The animations might not be a problem, but Blizzard would have to retexture every chestpiece and chest covering to make them wear player armor.
Their torso isn't smooth, it's a blocky carapace and their second set of arms does not potrude directly beneath their first set, but rather from their chest:
They'd have to comit to a ton of texture work to make sure every chestpiece, shirt and tabard looks actually good on them.
Which is on its own a lot of work, but even more so when you take into account every modern chest pieces that have ornate sleeves.
And sure, while they can pull a Dracthyr and just have them not show the chest gear, that would severly limit customizability.
Dracthyr do have a second form to accessorize, these guys would not.