This is mostly because of guys btw.
There are far more men who want a woman's cock up their ass than there are women who want to top, like the numbers aren't even close. Even ftms and lesbians prefer getting their holes stuff to stuffing the holes of others.

Personally I think this is because of some weird complex young men have with sexuality. They're straight but they're led to believe by society that wanting to top women is predatory/misogynistic/whatever so they get into pegging since its a way to have sex with women that's "safe".