ToB - where does it say that you can't Manyshot with a maneuver?

[Cogidubnus]

Exactly what the title says (ignoring the problem of most maneuvers being melee attacks for now).

[Zaydos]

Exactly what the title says (ignoring the problem of most maneuvers being melee attacks for now).

Well I don't know of any that could be in the actual book because they're either Range: Melee Attack, a boost (which is applicable), or a Full-Round Action.

For the others it's the same reason you can't use a Duskblade's Arcane Spellchanneling with a manuever; both are special standard actions that include attacks.

Manyshot is "as a standard action you may..."
Not "as part of an attack"

Maneuvers are "as a standard action make a..."

Which makes them incompatible.

[Morph Bark]

Because most maneuvers explicitly state how many attacks (note: not attack rolls) you get and are mainly melee. You also can't use Trip and Disarm in conjunction with maneuvers, unless they say so.

[Snake-Aes]

Exactly what the title says (ignoring the problem of most maneuvers being melee attacks for now).

As Zay said...maneuvers aren't part of attacks. They are independent actions. That's why most strikes either grant 1 attack or a full attack as part of their actions.

[Dragonmuncher]

To phrase it another way, Manyshot is sort of its own kind of maneuver. So, for the same reason you can't use two standard-action maneuvers at the same time, you can't use Manyshot and a ToB maneuver.

[ericgrau]

Manyshot is a standard action. It may not be done as part of another action, including other actions that grant attacks. Attacks are different from standard actions. You might be able to use a standard or full round action to get one or more attacks, but you can't use an attack to get a standard action.

[Starbuck_II]

Same reason you can't Spring attack and manyshot.
Action types.