I realize this may seem like a canned answer (and it is!) but it's still the right one: indie porn game devs make whatever it is they want to make/think will make for a good porn game. You're perfectly welcome to request stuff, but they don't have to do it.
So if you feel like there's a hole in the market (hehehe) for butt-themed porn, then the offered solution is either to make it yourself or commission somebody to. The other thing is I'm not really sure in what context you're looking for butt games: most of the IRL-porn games I've seen, for example, usually have at least one bigg butt lady if not several, and if you're looking for 2D animated games, you can do a lot worse than Dandy Boy Adventures. Others can recommend 3D animated porn games with big butts better than I can.
But if you want to consider why there are so many big titty games, consider the current IRL porn movie scene: loads and loads of nearly identical of-age ladies with basically nothing up top and nothing else going for them, aside from looking like they're fifteen and having butts, have flooded the market. Porn games, on the other hand, can have a bunch of thin big titted ladies without requiring expensive and invasive surgery. That makes it a niche that video game porn can fill much more easily than IRL porn.
Also? Butts are where poop comes from. And poop is smelly. Boobs, on the other hand, can make milk. Milk is food!
