It's because a good bard, even if they're putting out everything they have, is still at the mercy of RNGesus for the crits to sustain high DPS. Bard owns the field if you have group AOE pulls, but even with WM has trouble getting sustained single target DPS that is comparable to the other DPS classes. I've been keeping a close eye on my damage numbers and it's really disheartening that an ilvl 176 bard, even putting everything she has into her rotation, still comes out behind the WAR on the parse in a depressing 5th place in an 8-man fight.