Here's the thing with different Garrison types.
It is not a "negligible" amount of work. It's not "we'll do one or two every day and get it done in a few weeks."
If they were to make different models for every race and every upgrade, they would have to make over 900 building models. Frankly, I'd wager that's about as many as were added throughout the entirety of BC, WotLK, and Cataclysm combined.
Garrisons are conventional Horde/Alliance outposts. No reason for Blizzard to do that undue amount of work. It's not a realistic option.
Now, that being said... I really see no reason to have normal human/orc forts for our capitols out on some godforsaken rock, seeing as two different, far superior options already exist in-game and are for all intents and purposes basically completed.