Balance is the main reason I would assume.

Classes that have similar core mechanics = easier to balance.

Melee all have a Slashing/Blunt/Piercing buff, long DoT, short DoT, some sort of buff they have to maintain (Huton/GL/Blood)...

That being said, I don't disagree with BRD having less mobility than it had in 2.xx, but the WM mechanic (cast bars) makes the class clunky, moreso than Machinist which was built around that mechanic. WM was kind of an afterthought it seems, and to tack it onto a class that's built around off-GCD's and random procs/resets just feels awkward and doesn't mesh well.

I feel like the way around this would be to remove the cast bars, however under WM, give BRD reduced mobility (reduced down to walk speed) during the 1.5s duration that they are firing their arrows. This would allow off-GCD weaves such as Bloodletter, but reduce their mobility. Would make dodging AoE a little trickier but that's what stance dancing is for.