This would almost be like asking why a woman might like a man's abs, arms, or his chest. You do realize women are often staring at men's butts more than men are at women's.
I'm pretty sure you couldn't answer most of those.
I will tell you why I like a nice butt on a woman, others may have different reasons. I like a nice butt on a woman because it emphasizes her figure, a nice butt will indicate she takes care of her body (Or blessed with genetics) It is the core of her sensual sexuality. It tells men whether she is fertile or not (A primal instinct, really)
Some of the same things can be said about a woman's breasts(Although not as appealing as the butt), like if they're medium to large sized, but perky breasts, also means she's taking good care of her body-adding to her sexual appeal.
Which brings me to my next point, it's not ultimately the butt I find attractive it's her figure as a whole, it has to have that curvy but proportionate look.
And men are not attracted to every woman's butt, I'm only attracted to a few, I can tell if a woman doesn't work out, and get a rough idea of their diet.