The problem is, this basically forces them to only add crossbows to the drop table, as bows would not be usable. It makes the weapon pool for this new spec very very small, which is not good. Granted, the nature of personal loot means that large loot pools aren’t necessarily a problem, as long as the number of items relevant to any given spec on any given boss is manageable (generally 5 or fewer), and there isn’t a huge disparity between different specs and their available options.
The problem is the overlap. In Ny’alotha, for example, there are 2 agility daggers (which only Assassination and Sublety can use), and 3 weapons Combat could use (a sword, a fist, and a mace). Havoc similarly has 3 weapons (the same sword and fist, plus a warglaive). Warriors and DKs have 3 weapons (polearm, 2h sword, 2h axe).
There’s a total of 5 caster weapons: a wand, a staff, a 1h axe, a 1h sword, and a 1h mace. Of the casters, druids cannot use the axe or wand (3 weapons), mages cannot use the mace or axe (3 weapons), monks cannot use the wand (4 weapons), paladins cannot use the staff or wand (3 weapons), priests cannot use the axe or sword (3 weapons), shamans cannot use the sword or wand (3 weapons), and warlocks cannot use the axe or mace (3 weapons). So monks are slightly better off, but everyone else balances out.
But in this case, this spec would exclusively use crossbows, which overlaps with hunters, meaning either only crossbows would be on the loot table, or hunters would always have more weapons options than this new spec. It’d be like if this new spec was designed to use daggers and also swords, we’d always have more options than sin/sub rogues.
Now, if they were a different type of crossbow, specifically a 1h variant exclusive to this new spec, this issue would disappear, but then Blizzard would have to devote artist resources (and item table records, though that’s unlikely to be an issue) to these new weapons exclusively for one spec in the game. That’s asking rather a lot just so you can get in your D3 fantasy fix in WoW. I doubt that level of investment requirement is something Blizzard would be willing to entertain.