The issue with things like the 2B butt is that the game's not really made for them. Like if you sit down in a chestpiece that has a skirt, the butt clips through the skirt instead of playing nicely:

the game's current animations just aren't made to support them, and would have to be changed a lot to accomodate them nicely. The 2b set is kind of janky anyway (like how the semitransparent cloth on the boob window is really just dark peach color, so chars with different colored skin seemingly have a different skin color just on the top of their boobs), it's just in there as-is because it's a single fanservice item.

Having certain body types be locked to a specific piece of gear kind of sucks, because it means that if you really want that body type then you're stuck with a specific piece of gear forever, until you complain enough for SE to put in another piece of legwear with extra butt in it. But then you're still just stuck with 2 options instead of 1.