Steatopygia was common in some hunter gatherers, including European ones, and still persists today among San Bushmen.

Why did it disappear?

First we have to understand It's a fat storage mechanism. But more importantly, the lower body stores more polyunsaturated fat, which slows the metabolism, which enables even longer survival than normal fats. A fat ass literally keeps you alive longer than a fat belly.

Seemingly unrelated: It was also common during the first days of farming, for men to go out and hunt while the women stayed behind and grew up malnourished. We know this from skeletal indicators of development. Basically the guys would go out and hunt, but they wouldn't bring anything back.

Combine both of these facts, and the truth becomes apparent: steatopygous tribes were those that didn't feed their women. While slender tribes were better providers.