Asking for a friend... what is the original reason for druids' metal aversion?

[Arkhios]

Seriously though, let's admit it: I've just forgotten and couldn't find it quickly enough from the depths of internet. All the top searches that spring up are the ones related to 5e and exploding druids.

So, let's humor the question/topic anyway. Let's say there's a new player who wonders "Why, though? Druids can use weapons with metal parts, so why not armor or shields?"

The real question here is: What was the original reason in that one past edition (I've also forgotten which one) and the origin for this idea?

[KorvinStarmast]

To make them different from regular clerics. I can't offer you the cite from Eldritch Wizardry, since my e version of that got corrupted.
Druids began as a sub class of clerics.

Gygax and the rest of the team decided that, like a wizard NOT wearing metal armor, druids would not either.

See also this:
https://rpg.stackexchange.com/a/70528/22566

I have made a variety of posts on this topic over the years, and have been hammered by the mods for them so I can add nothing further.
The "druids and armor" noise threads for 5e are now in my past.
I strongly suggest that you head to dragonsfoot and ask there. There is a whole crew of old schoolers who know this topic well.

[Amnestic]

I believe the original 'explanation' was

The more powerful druidic spells, as well as their wider range of weaponry, make up for the fact that druids are unable to use any armor or shields other than leather armor and wooden shields (metallic armor spoils their magical powers).

Which doesn't really tell you much. Why does metal armour spoil their magic powers?...because it does! Okay.

3.5e mentioned that "the armours of a druid are restricted by traditional oaths", that "all other armour is prohibited", and that "Druids avoid carrying much worked metal with them because it interferes with the pure and primal nature that they attempt to embody."

Which doesn't make a whole lot of sense since metal is a part of nature, and would have to be worked just as much as tanned leathers to be useable as armour, but that's what they said.

In 5e, Sage Advice Compendium suggested that "Druids don’t lack the ability to wear metal armor. They choose not to wear it. This choice is part of their identity as a mystical order. Think of it in these terms: a vegetarian can eat meat, but chooses not to."

The armour thing has always been at odds with druidic weapons including sickles, which got expanded into scimitars, which is more 'real life/other media' druidic trope of harvesting stuff with sickles. Why are metal weapons fine but metal armour isn't? Just...because! There's likewise no restriction on metallic jewellery (rings, amulets) or magical helmets, nor why they're cool with carrying and using a bunch of worked metal coinage.

People will provide some explanations for this, but ultimately it doesn't make logical sense and ends up being arbitrary.

So far as I know 5.5e has gotten rid of it entirely and just adjusted their standard armour proficiencies instead, so that if you do pick up heavy armour proficiency elsewhere you can indeed be a heavy armoured druid.

I'm not against the idea that druids shirk metal for metaphysical/supernatural reasons, but if so they should go whole hog and shirk all metal - no metal weapons, no metal rings, and no carrying gold pieces, with class features buffed to reflect this relatively significant restriction, or the availability of 'alternative' materials ("Ironwood" - strong as iron, made of wood!) that will serve in metal's place.

[Sorinth]

From my old AD&D PHB it says "The more powerful druidic spells, as well as their wider range of weaponry, make up for the fact that druids are unable to use any armor or shields other than leather armor and wooden shields (metallic armor spoils their magical powers)."

My personal opinion is that they felt metal armour was closely related to industry, and especially in the 70s industry and nature were considered to be opposites of each other. But they were allowed metal weapons because of the Celtic influences.

[Grim Portent]

I think the cultural portrayal of celt's as unarmoured compared to their contemporaries* probably influenced it at least a bit. We know very little about real druids except what little bits were written about them by the Greeks and Romans, or which survived in folklore, and there's a few letters from Roman generals that talk about Celts fighting without armour, or even completely naked, and the relative scarcity of chainmail found in their graves kind of all just leads to this common depiction of them fighting in clothes or bare chested, or even naked head to toe.


This is a weird thing to cite, but think about Asterix and Obelix, and the portrayal of the Celtic Gauls in it. While not aiming for accuracy by any means, it's not actually all that out there in terms of how Celts are often depicted in terms of attire. Druids being heavily inspired by the same general aesthetic, plus some nature vs man type theming, would make prohibiting armour a natural way to make them 'Celtic' in a way that Cleric isn't.

*True in the case of the Romans admittedly.

[KorvinStarmast]

But they were allowed metal weapons because of the Celtic influences. Explicitly because of sickles. See the link in my post.
(Also the whole Law = civilization+tech and Chaos = wilderness+lowtonotech is a contributor to the paradigm).
But as Gygax said:

The primary appeal of the Druid class from a creative standpoint is that the Romans were so thorough in destroying them and their religion that we know virtually nothing about either.

[sithlordnergal]

You know, I'm kind of glad they're getting rid of the armor restriction. Its actually kind of funny, I'm fine with the 3.5 Druid not being able to wear metal armor because if they wear the armor they lose all of their Druid abilities and spells for 24 hours. But the half baked RP reason in 5e makes me irrationally angry because it makes no sense. Either get rid of it, or give an actual consequence. XD

[Kurald Galain]

We know very little about real druids except what little bits were written about them by the Greeks and Romans, or which survived in folklore,
Sure. We do know, however that scimitars (a) look absolutely nothing like sickles, and (b) are from a completely different region of the world (and, yes, (c) made of metal). The scimitar being "the" standard druidic weapon is utterly ridiculous no matter how you slice it.

[oudeis]

To follow up on Sorinth and Korvin's points, it's also worth noting that as residents of the American Upper Midwest, Gygax et al would be well-acquainted with the environmental consequences of the steel industry- vast landscapes devastated by iron mining, toxic air and water pollution from urban mills, etc.

[Unoriginal]

I think the cultural portrayal of celt's as unarmoured compared to their contemporaries* probably influenced it at least a bit. We know very little about real druids except what little bits were written about them by the Greeks and Romans, or which survived in folklore, and there's a few letters from Roman generals that talk about Celts fighting without armour, or even completely naked, and the relative scarcity of chainmail found in their graves kind of all just leads to this common depiction of them fighting in clothes or bare chested, or even naked head to toe.

[...]

*True in the case of the Romans admittedly.

The Gauls invented mail armor.

[Sorinth]

To follow up on Sorinth and Korvin's points, it's also worth noting that as residents of the American Upper Midwest, Gygax et al would be well-acquainted with the environmental consequences of the steel industry- vast landscapes devastated by iron mining, toxic air and water pollution from urban mills, etc.

Nowadays instead of a limitation of metal armour it would be the druid refuses to use any armour or weapons unless they were were made from ethically sourced materials from smiths that used sustainable crafting practices. If the druid wanted to wear Ankheg Half-Plate they would first need to ensure that it was from free range Ankheg's :smallbiggrin:

[Theodoxus]

Metal negated magic, be it druidic or wizardry. Clerics were given special dispensation because (checks notes) their gods where ok with it.

Scimitars, I totally agree are dumb on the whole. Use a shillelagh; heck cast shillelagh on a shillelagh and call it a day. The only advantage scimitars have in 5E is being finesse, but that's not as good as using your Wis to attack. I wish clubs were just called shillelaghs. I hate the aesthetic of Captain Caveman style meat clubs.

TBH, druids as a thing should just be Nature Clerics in 5E. The whole wildshape schtick is a mishmash of silly lore turned into a Frankenstein love child from disparate game systems, video games, MMOs, etc. so that the idea of Druid as nature priest has succumbed to 'Ranger that can transform into a bear' that makes Beorn sad.

Time to retire the name and rebuild the wildshaping class into something that isn't a primary caster, so the wildshapes can actually be fun and useful (a bit like the druid from Honor Among Thieves). Then, they can stick with the whole 'non-metal' armor thing, because 'metal can't be subsumed into the new form, for reasons.'

[Arkhios]

Time to retire the name and rebuild the wildshaping class into something that isn't a primary caster, so the wildshapes can actually be fun and useful (a bit like the druid from Honor Among Thieves). Then, they can stick with the whole 'non-metal' armor thing, because 'metal can't be subsumed into the new form, for reasons.'

My wife is about to start her own game at her workplace as a weekly recreational activity for chosen clients (mostly severely disabled people etc.) and she made that exact decision: druid in her game won't be a spellcaster, but rather a wildshaping class that borrows from other classes' abilities depending on the chosen form. e.g. a bear form deals extra damage akin to the barbarian's rage or dueling fighting style.

[Grim Portent]

Sure. We do know, however that scimitars (a) look absolutely nothing like sickles, and (b) are from a completely different region of the world (and, yes, (c) made of metal). The scimitar being "the" standard druidic weapon is utterly ridiculous no matter how you slice it.

Apparently no one could think of any other kind of curved blade weapons at the time, so they just stuck with scimitars. Not that Druids would have used sickles to fight anyway, I imagine they would have used a straight sword if they were feeling fancy, or a spear. Assuming they used weapons at all and weren't pacifists or something.

Dumb D&Disms don't shift easily, and grow into their own weird little fictional oddities that continue to exist because people get attached to mistakes.


The Gauls invented mail armor.

The Celts invented a whole lot of stuff, they still got typecast as barbarians in a lot of pop-culture/pop-history stuff, and its probably the cultural aesthetic of bare-chested guy with a big moustache that influenced things like the D&D Druid more than accurate historical stuff.

Well the portrayal of druids as long bearded nature wizards that came up long after they died out probably played into it more than the buff and bleached celtic warrior, but one sort of grew from the other over retellings.

[Theodoxus]

There seems to be a bit of confusing overlap between the Celts and the Picts in this thread. The Celts were on par with their Nordic cousins, technologically, though not quite as invade-y. Probably helped that the lived in more temperate climes than the Norse. The Picts were pretty ruthless as a people, but lacked the tech to hold their own against the Romans. It's the Picts who ran around making war in the nude (and woad), and where a lot of modern druidism stems from (helps (or doesn't, depending on your view) that King Arthur (the 2004 movie) played up the Pict/Druid idea, with Merlyn being a Pictish druid instead of a wizard.

Not that it matters in the grand scheme of things...

[Grim Portent]

There seems to be a bit of confusing overlap between the Celts and the Picts in this thread. The Celts were on par with their Nordic cousins, technologically, though not quite as invade-y. Probably helped that the lived in more temperate climes than the Norse. The Picts were pretty ruthless as a people, but lacked the tech to hold their own against the Romans. It's the Picts who ran around making war in the nude (and woad), and where a lot of modern druidism stems from (helps (or doesn't, depending on your view) that King Arthur (the 2004 movie) played up the Pict/Druid idea, with Merlyn being a Pictish druid instead of a wizard.

Not that it matters in the grand scheme of things...

Picts were a subgroup of Insular Celts, probably related to the Britons in England,* and by extension to the modern Welsh and Bretons. Celt is broad group, like Germanic, of which there are many subdivisions. Celtic subgroups that I actually know off the top of my head are the extinct Gauls, Britons, Picts, and whatever the Iberian and Italian Celts were called, and the still living Irish, Scottish, Welsh, Breton, Cornish and Manx people. Maybe the Orcadian/Shetlanders, though I'm not clear on if they're Celtic or Nordic off the top of my head, they're definitely a bit of both but I'm not sure which side they fall on.

But yes, you are correct in that the Picts are the ones the Romans wrote about being naked barbarians, but they were probably making an arbitrary distinction between the Britons and the Picts that wasn't actually there, and generalising about all the tribes outside their control to dismiss them as barbarians, neither group would have been homogenous and there would have been overlap, pre-Romanised Britons were probably extremely similar to the groups labelled Picts later on, especially as you went further North. There is some evidence that Gauls were into tattoos as well, which is probably what the Romans meant when they talked about the whole blue paint thing.

*I used to think they were Gaelic until a few years ago, but consensus seems to be that they were Brythonnic, which makes sense in retrospect, shared landmass and all, us Gaels nipped over from Ireland and took over Scotland one way or another.


EDIT: I think a lot of the basic image of the druid was established long ago, there's paintings of druids as basically prototypical woodland sorcerers with long beards that go back a ways. At least as far back as the 1700s I think.

[KorvinStarmast]

Sure. We do know, however that scimitars (a) look absolutely nothing like sickles, and (b) are from a completely different region of the world (and, yes, (c) made of metal). The scimitar being "the" standard druidic weapon is utterly ridiculous no matter how you slice it.
Suggest you read the link I posted in my first response. It's about making a thing for a GAME. In AD&D 1e Unearthed Arcana, they added the Kopesh as an OK druid weapon because there was a line of thinking that the Druids of the British Isles/Gaul had a connection with the Ancient Egyptians ...(There's a trilogy about "The Seedbearers" which covers some of that. Power of the Serpent, and Twilight of the Serpent are the other two books in the trilogy ... Wessex, etc).

Take a look at some of the interesting Speculation on how the alignment of the Great Pyramids and Stonehenge (from a certain perspective) is not an accident. Back in the 70's when this convention was adopted, that kind of thing was very current and widely circulated among geeks and wargamers. Wargamers are the hobbitical ancestors of modern gamers. (And that is also the time frame where von Danekins "Chariots of the Gods" (https://m.media-amazon.com/images/I/513AcYigLiL._SL350_.jpg)was alive and well among that similar population of geeks and gamers).

----------------------------------------------

Hobbitical has naught to do with hobbits, but rather with hobbies. Wargaming, and gaming, are hobbies.

--------------------------------------------------

Yes, I just invented that neologism. Feel free to use it. :smallyuk:

[Lord Torath]

Metal negated magic, be it druidic or wizardry.Citation needed.

Metal armor does not provide general protection against spells in any version of D&D that I'm aware of. Spells with metal components can still be cast by wizards/druids. The 2E PHB explicitly said metal does not interfere with magic.

[Theodoxus]

There is a significant difference between channeling magical energy while essentially wrapped in a faraday cage, and being struck by magical energies taking the form of fire, lightning, or acid.

It harkens back to the cold iron allergy of fey. Though it's very interesting that by AD&D, elves could wear armor and cast spells if they were multiclassed Fighter/Magic-users.

The original idea being that magic was taught by magical fey creatures, and thus, one couldn't wield magic while encased in such a non-magical-energy-conducting uniform like a suit of armor.

Obviously, while the game lore evolved, such concepts went by the wayside in search of more fun ideas, I'm sure.

You want citations, you'll need to look further into the web than 2nd Ed.

[Lord Torath]

There is a significant difference between channeling magical energy while essentially wrapped in a faraday cage, and being struck by magical energies taking the form of fire, lightning, or acid.Very few spells actually channel elemental energy. Most don't. Hold Person. Charm Person. Levitate. Tongues. Teleport. Polymorph. Telekinetics. ESP. Power Word Stun/Blind/Kill. Enlarge/Reduce. Strength. Maze. I could go on. None of those involve fire, lightning, acid, cold, or any other damaging energy, and none of them are affected at all by the target wearing metal. There's nothing stopping Explosive Runes from being written on a metal surface. Even back in 1E, it was explicitly called out that the metal in armor was not the reason wizards couldn't wear armor.

[Chronos]

Those old editions where druids originated (in the game) also had clerics only being allowed to use bludgeoning weapons, nothing sharp. Why? Because they just do. Just like druids.

[Grim Portent]

Those old editions where druids originated (in the game) also had clerics only being allowed to use bludgeoning weapons, nothing sharp. Why? Because they just do. Just like druids.

IIRC that rule is based on a myth about priests using blunt weapons to bypass a rule forbidding them from spilling blood. I even remember reading that myth in my Horrible Histories books as a child.

My understanding is there was a canon law forbidden clergy from spilling blood, but priests just did it anyway and seem to have mostly gotten away with it. There's not really any evidence they limited their weapon choice to minimise blood spilt in combat.

[KorvinStarmast]

My understanding is there was a canon law forbidden clergy from spilling blood, There was also a rule about banning crossbows as weapons. Its enforcement was, however, uneven at best.
By the late medieval period, edicts such as the Second Lateran Council of 1139 sought to ban its use among Christians against their fellow believers.

[Amechra]

The Celts were on par with their Nordic cousins, technologically

The La Tène material culture (https://en.wikipedia.org/wiki/La_T%C3%A8ne_culture) predates anyone who spoke Old Norse by a full millennia, so... I'm not sure if this is you praising the Celts or dissing the Norse. :p

[Kurald Galain]

The original idea being that magic was taught by magical fey creatures, and thus, one couldn't wield magic while encased in such a non-magical-energy-conducting uniform like a suit of armor.
Wizards aren't prohibited from wearing metal armor, but are prohibited from wearing armor period.

I'm pretty sure back in 1E this was purely for balance reasons, and later editions give their own spin on it. But if metal blocked magic, then by that same logic you should wield a spiked chain and wrap that around enemies to keep them from casting.

[Arkhios]

Wizards aren't prohibited from wearing metal armor, but are prohibited from wearing armor period.

I'm pretty sure back in 1E this was purely for balance reasons, and later editions give their own spin on it. But if metal blocked magic, then by that same logic you should wield a spiked chain and wrap that around enemies to keep them from casting.

While I certainly appreciate Theodoxus' evocative writing, I can't help but feel that they are a bit more colored with flavor, than facts and citations from the books or the original designers of said books; I mean, they're hinted between the lines, of course, but even I would appreciate seeing solid citations from said books, because, for one I don't own the older books in any form whatsoever (in fact, currently the oldest rules I have at my disposal is only the 3.5 PHB)

[KorvinStarmast]

Wizards aren't prohibited from wearing metal armor, but are prohibited from wearing armor period. Yes. That goes back to OD&D / three brown books.

I'm pretty sure back in 1E this was purely for balance reasons Not wholly, since an elf could be a fighter/magic user, but yes. Each role was supposed to have it's own niche. And Divine Casters could wear the armor a Fighting Man could.

But if metal blocked magic, then by that same logic you should wield a spiked chain and wrap that around enemies to keep them from casting. Sure, why not? Arcane casters are OP anyway. If you broke a round down into segments, some spells didn't go off for a few segments, giving the martials in opposition a chance to disrupt the casting of the spell.

While I certainly appreciate Theodoxus' evocative writing, I can't help but feel that they are a bit more colored with flavor, than facts and citations from the books or the original designers of said books; I mean, they're hinted between the lines, of course, but even I would appreciate seeing solid citations from said books, because, for one I don't own the older books in any form whatsoever (in fact, currently the oldest rules I have at my disposal is only the 3.5 PHB) The original authors were not subjected to the volume and rate of nitpicking that internet era authors have been. (Althorugh plenty of nit picking went on in various newsletters of magazines like dragon, those didn't flow at the speed of electricity).

[Sorinth]

For wizards and armour, my AD&D PHB says it's because of a lack of training time since they've had to spend their whole life on study there was no time to train with armour. In 2E PHB they expanded it a bit as both a lack of training and the armor interfering with the movement required to cast spells, they also specifically call out the whole metal interfering with casting spells as a myth that commoners believe but wizards know to be untrue.

[Segev]

Incidental anecdote: in a 5e game, the fact that druids "won't" (rather than "can't lest their powers stop working," or some other consequence-based penalty for doing it) benefited my druid PC, once.

He was whammied by a suggestion spell that had him cooperating with going to meet the people who'd sent a bounty hunter (who could cast suggestion) to kidnap him. The bounty hunter tried to set as a condition of meeting them (which the suggestion compelled my PC to do) putting on chain mail, with which he is not proficient and thus which would have kept him from casting any spells.

Since druids "don't" wear metal armor, that wa not 'reasonable' under the terms of the suggestion spell. He refused, and proceeded to infiltrate the building via his own means rather than walk in in metal armor.

[MonochromeTiger]

While I certainly appreciate Theodoxus' evocative writing, I can't help but feel that they are a bit more colored with flavor, than facts and citations from the books or the original designers of said books; I mean, they're hinted between the lines, of course, but even I would appreciate seeing solid citations from said books, because, for one I don't own the older books in any form whatsoever (in fact, currently the oldest rules I have at my disposal is only the 3.5 PHB)

That is a very polite phrasing for "you keep saying things as fact, please cite your sources."

That said the way things were written originally does leave lots of vague points and lack of justification, and at the time that was fine because like KorvinStarmast points out there may have been some critics who got annoyed about it but it wasn't nearly the instant barrage from all sides that leaving such massive blanks in the rules causes now. What people now expect some lore reason for, some bit of fluff justification to make sense of things, back then could get away with being a flat "they do this because we say they do."

The point wasn't to have everything supported by everything else it was just to have enough detail that the game was playable. Some of that detail was down to early ideas of game balance and distinctness, something that many people are still arguing back and forth about several editions later. Some is down to a very shaky understanding of the source material due to lack of available resources on the matter. The no metal armor thing most likely comes down to the former while other things like the very limited weapon availability and the proficiency for scimitars or khopeshes or whatever each edition chooses to throw in because "it's curved, that makes it like a sickle" come down to the latter. Those two reasons get mixed up constantly and even now, when we do actually have a better understanding of things than back then, we've still got plenty of people who will try to justify the first reason by appealing to the second without bothering to so much as read a wiki article let alone do actual research to see why that doesn't work.


Incidental anecdote: in a 5e game, the fact that druids "won't" (rather than "can't lest their powers stop working," or some other consequence-based penalty for doing it) benefited my druid PC, once.

He was whammied by a suggestion spell that had him cooperating with going to meet the people who'd sent a bounty hunter (who could cast suggestion) to kidnap him. The bounty hunter tried to set as a condition of meeting them (which the suggestion compelled my PC to do) putting on chain mail, with which he is not proficient and thus which would have kept him from casting any spells.

Since druids "don't" wear metal armor, that wa not 'reasonable' under the terms of the suggestion spell. He refused, and proceeded to infiltrate the building via his own means rather than walk in in metal armor.

And this is kind of the endpoint of the entire vague wording issue. It's kept in as a legacy decision, occasionally something is thrown in to try justifying it but that just raises more issues than it solves so eventually it loops back to a vague "they just don't." Meanwhile as far as actual games go it becomes one of three things, a restriction put on player characters, something the table just ignores, or something to exploit the wording of.

You can pull off some things in game provided your DM agrees with your reasoning but otherwise it's just a thing that's there. Why? Because reasons.

[JonBeowulf]

Depends on how far you want to go back to find the original reason. In BECMI, it was simply to create a character class that was similar to clerics but entirely nature-based. I figure the rationale went something like this:

I think a nature-cleric-thing would be cool. (sets about making a list of really cool nature cleric spells)
Hmm... this new class needs to be similar to clerics but also different.
Clerics can't use edged or pointed weapons... how about druids can't wear metal armor or use metal weapons? (group goes "hells ya!")
Justification: Working with metal requires industry far more advanced than working with wood and leather does. Druids are focused on the balance of nature while industry is focused on bending nature to the will of man (or something like that). Druids will use metal tools because they can use those tools to care for nature but they will not use metal weapons or armor.

Heck, you couldn't even be a druid until you reached level 9 as a neutral-aligned cleric. Quite a feat back in those days.

[GeoffWatson]

Because Getafix from the Asterix comics (the main source for Druid information) didn't wear armour.

[2D8HP]

What it said in the 1976 Eldrich Wizardry supplement was:

Druids are able to employ the following sorts of weapons: Daggers, sickle or crescent-shaped swords, spears, slings, and oil. They may wear armor of leather, and use wooden shields. They may not use metallic armor.

In the 1978 Player’s Handbook it said:

Druids are unable to use any armor or shields other than leather armor and wooden shields (metallic armor spoils their magical powers).




Citation needed.

Metal armor does not provide general protection against spells in any version of D&D that I'm aware of. Spells with metal components can still be cast by wizards/druids. The 2E PHB explicitly said metal does not interfere with magic.


Very few spells actually channel elemental energy. Most don't. Hold Person. Charm Person. Levitate. Tongues. Teleport. Polymorph. Telekinetics. ESP. Power Word Stun/Blind/Kill. Enlarge/Reduce. Strength. Maze. I could go on. None of those involve fire, lightning, acid, cold, or any other damaging energy, and none of them are affected at all by the target wearing metal. There's nothing stopping Explosive Runes from being written on a metal surface. Even back in 1E, it was explicitly called out that the metal in armor was not the reason wizards couldn't wear armor.


Wizards aren't prohibited from wearing metal armor, but are prohibited from wearing armor period.

I'm pretty sure back in 1E this was purely for balance reasons, and later editions give their own spin on it. But if metal blocked magic, then by that same logic you should wield a spiked chain and wrap that around enemies to keep them from casting.


IIRC in an issue of The Dragon there was a lengthy essay explaining that the close by mass of metal armor did interfere with the arcane magic employed by magic-users (but not cleric’s magic), something to do with magnetism?

I can’t recall why elves were exempt (maybe elves used special alloys as was mentioned in the foreword to Poul Anderson’s 1970’s revision of his The Broken Sword?).

I don’t remember seeing any later similar reasoning for metal interfering with spell casting, I think that it was just a one off essay.

[KorvinStarmast]

IIRC in an issue of The Dragon there was a lengthy essay explaining that the close by mass of metal armor did interfere with the arcane magic employed by magic-users (but not cleric’s magic), something to do with magnetism?
I remember that Poul Anderson's works had something in it about the degaussing effect of iron/steel on magic. It might have been in Operation Chaos or in Three Hearts and Three Lions. Or both. Three Hearts and Three Lions was one of the books (Appendix N, yadda yadda) that formed the pulp/SF context of early D&D.

[Segev]

The ultimate reason is the same as why wizards can't wear armor (under 'normal' circumstances ): aesthetics.

The druid is supposed to be wearing hide and chitin or the like, while his look is still fine if he wields a sickle.