D&D will increase it's Cadence of Releases

[Theodoxus]

Because 2e and 3e is very out of date. In most cases, the timeline has moved on. In all cases, world specific mechanics aren't easily adaptable. But unless you've got lots of money, spending big bucks for what's effectively a one-time read through on the lore portions doesn't appeal to me personally. I learned that with SCAG.

for adventures, changes apply with spades for mechanics. Paying someone else to adapt them is a huge time saver, and well worth spending money. For example there are several very well done conversions of the B-series modules out there.

One unfortunate side effect of running old modules is figuring out how to handle 5e PC's incredibly rapid level advancement.

In 3E there were 2 books (Lords of Madness and Draconomicon) I bought for Lore.

I wanted a deeper understanding of certain creatures. For Dragons I did have plenty of options outside D&D, but always found room for at least 1 more. They each had new takes or expanded on existing lore to flesh out this iconic creature. For Aberrations, many of them were invented by D&D, so there were no sources outside D&D. I learned a lot about Illithids from Lords Of Madness.

Campaign Setting lore books on the other hand had to sell me on the additional mechanics. I only gained some appreciation for a setting after it had hooked me with some other bait. That said, I became interested in Sigil enough that a sufficient up to date lore book on that might tempt me. Sigil has always been a place of shallow sources using contradicting theories, so a modern summary would be nice.

In both these cases, you're following someone else's lore though. Not even the original creator of said lore in most (all?) cases. Just a ghost writer who said "I think this is cool."

Now, I get if you're rushed for time, or don't have the inclination (for whatever myriad reasons) to update old lore to new for your take on the setting... so reading someone else's work and then culling it of ideas that don't mesh with your take makes sense, especially if you can afford to dish out the cash for it. But take this excerpt for example. It's 3E lore. How much change has happened in 2 editions? What can't be run straight out of the FRCS book? (and before a claim of cherry picking, please know I literally just picked a random page that would copy/paste well...)

Spoiler: The Moonsea - Forgotten Realms Campaign Setting

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The Moonsea
Capital: None
Population: 1,745,280 (humans 69%, orcs 10%, half-orcs 6%, halflings
5%, dwarves 5%, ogres 2%, gnomes 2%)
Government: Independent city-states usually under oligarchic rule
Religions: Bane, Cyric, Loviatar, Mask, Talos, Talona, Umberlee
Imports: Food, textiles
Exports: Forged metal, furs, gems, lumber, raw minerals, slaves
Alignment: N, LE, LN
The Moonsea proper is a deep natural lake located north of the Dalelands and west of the Vast, connected to Sea of Fallen Stars by the shallow, swampy River Lis. The name “Moonsea” applies to both the lake and its environs, which stretch south as far as the mouth of the Lis where the river flows into the Dragon Reach, west to the Dragonspine Mountains, north to the frigid steppes of the Ride, Thar, and the Tortured Land, and east to the Galena and Earthspur Mountains.

The Moonsea region holds a huge amount of mineral wealth for those hardy enough to brave peril to win it. The area is infamous for its bleak landscapes, harsh winters, ravaging dragons, pirates, and rapacious local governments. Some of the most aggressive cities in Faerûn rise from the Moonsea’s shores: Hillsfar, Mulmaster, and Zhentil Keep. Wherever a government exists in the Moonsea, it rules with an iron grip. Tombs, ruins, and monster lairs abound in the Moonsea, as do complex intrigues and double dealing tyrants. The battle cry of the area is well chosen: “Dare and Beware!”

LIFE AND SOCIETY
The Moonsea is a frontier, with a frontier mentality. The area stands as a buffer between the elven lands to the south and the darker, more sinister lands of the Ride and Thar, home of dragons,
giants, and ogres. Cities rise quickly, built on nerve and wealth, only to fall in wars or raids and then to be rebuilt once more. Only the strongest and most savage prosper in the lands of the
Moonsea. Evil folk who ruthlessly control their lands rule the greatest cities. The people of Hillsfar, Melvaunt, Mulmaster, and Zhentil Keep are accustomed to cruel lords, for this is a cruel land. Better
to support the devil one knows than confront one of even more deadly and fell power. The lesser cities of Elventree, Phlan, and Thentia are less overtly evil but have a strong, independent, almost
chaotic nature.

The people of the Moonsea view the rest of world and even the other Moonsea cities with suspicion and distrust. In return, the rest of Faerûn sees the people of the Moonsea as unfriendly, sullen, dispirited, crafty, and most of all dangerous. In truth, they are no more universally evil than the well-respected Dalesfolk to the south are universally good. But travelers here are best forewarned to
remain wary nonetheless.

MAJOR GEOGRAPHICAL FEATURES
The Moonsea itself is the dominant feature in the Moonsea area, along with the fast, cold rivers that feed and drain it and the mountains that border it. North and west of the Moonsea, the intrigues
of the cities fall away to the brutal dangers of the environment and native inhabitants who don’t care for having their lands invaded.

Dragonspine Mountains: The western end of this high mountain range marks the western extent of the Moonsea area. Red and white dragons lair among the higher peaks. Goblins, orcs, and giants
occupy the lower peaks and the valleys. Mines in the western half supply minerals to Zhentil Keep, while the cities of Melvaunt and Hillsfar draw resources from the eastern half.

The Flooded Forest: This fetid swamp surrounded by boggy woodland lies between the river Lis and the Earthspur Mountains. Once an arm of Cormanthor, the area has sunk into a low bowl over the course of the past hundred years or so. The trees of the forest still stand, but have been dead for more than a century. Many are ready to fall at the slightest touch. Explorers face as much danger from falling trees as from wandering monsters. Hanging mosses and mushrooms are everywhere.

Denizens of the swamp include lizardfolk, black dragons, a number of fungus creatures, and carnivorous plants. Some of these creatures are not native to the area, giving rise to claims that someone or something is stocking the swamp to keep others away. The depths of the swamp hold the hidden lairs of brigands and pirates (especially near the southern end). Rumor has it that the
swamp also hides ruined temples of Gruumsh, Moander, and Bane. With the return of Bane, it’s a good bet that any temple to that deity hidden in the swamp has been secretly repaired and reopened, perhaps with an eye toward extending Bane’s influence south into the Vast.

The Moonsea: The depths of the Moonsea’s cold, clear, and purplish waters are said to have connections to underground seas and to the Elemental Plane of Water. No permanent islands break its surface, but rumors persist of isles that rise from the deeps on certain nights of the year.