Special genetics play into how humans at different locations evolve. Studies show that our women became associated with wider gluteal features & gynoid adiposity simply because it was what was desirable among the men around them over hundreds of thousands of years.
And who could blame them? Gynoid adiposity (body fat that forms around the hips, breasts and thighs) have been positively associated with higher fertility outcomes
Over time, the flatter butted people moved on from the motherland elsewhere (Europe, Asia etc).

Over time, the flatter butted people moved on from the motherland elsewhere (Europe, Asia etc).
Simply because they weren’t as desirable to the locality & this played into our disparate ideals of beauty; why the typical Caucasian would say sexy is a tight butt but we like it buxom buxom. It’s not our fault 
This is also why you’d see the odd white or Asian with big butt.

This is also why you’d see the odd white or Asian with big butt.
Because mother Africa, in its usual generosity, didn’t take out all the bum bum making genes from the pool and it’d show up elsewhere. I swear all we do is give and give and give 
Sarah Baartman & many like her were demonized for their Steatopygia but now everyone else wants it

Sarah Baartman & many like her were demonized for their Steatopygia but now everyone else wants it
By the way, the reason men are more attracted to women with big butts and boobs is deeper than aesthetics, it’s science. Studies have linked fat deposition around the thighs, butt and breast with higher fertility (most probably because it signifies higher estrogen levels).
So men that go after those features aren’t just being vain, they’re moved by something deep beyond their conscious which equates those features with high fertility, scoring her higher as a mating partner. Yes, even without their knowledge.