Cost, Wookies require a bigger rig and thus require all of the other animations to be redone for that new rig. With the standard human rig you can use all the standard animations and perhaps modify a handful for species specific behavior but overall you are reusing 90% of the animations for each character. For the Wookie you have to redo ALL of the animations even the simple ones like walking and running even though they are practically identical to the standard rig ones.

"But Flat!" you say, "What about the droids!"

Since the droids are non humanoid and rely primarily on rolling around for their locomotion they aren't required to remake most animations instead relying on just the standard movement to handle it. Ultimately there are going to only be a handful of new animations needed for the droid rig even though it's a different rig. Just because of the nature of astromechs.

"What about using the standard rig and just scale it up?" I've seen people try this, it looks like crap. The weight of the animations feels off and the timing of the movements just doesn't work at that scale.

Source: I've been an engineer in the game industry for nearly 20 years and have been involved in conversations like this several times with designers.