I think it's perfectly reasonable to vent about the lack of CAST parts. Sure they might sell less or be in less demand, but it still sucks to be treated unfairly. I don't see why they can't let up on the cute girls a bit to give everyone something.
The reason is because it's much easier to create something that all races can wear than it is to go out of one's way to make something only one race can wear. And considering the ratio of male to female characters it does make more sense (if you're lazy) to focus much more on a single gender as well. So simply put it's just SEGA JP being lazy and half assing things, but considering this is SEGA we're talking about that is to be expected (not that it's right or anything).
We have existing parts which are unreleased in Global, enough to release one set per week and need about a year and half to release them all. There's nothing to create from scratch.