Because the ONLY job you see raids calling out for are Bards. Never or hardly see any other FC/groups call out for any other particular class but bards...

But It's mainly due to a few good reasons.

*Songs
*Range DPS
*Great DPS
*Silences
*and the fact that Melee get F_cked with any boss in game thanks to aoe/cleave dmg.

Mechanics of the game are hurting the classes in the game.

/truestory