One thing that researches have learned over the years, is that people are 'naturally' attracted to 'proportions'. Thus, someone who's facial features match a specific ration generally receive higher pay, are considered more attractive, and are both more popular and, generally, happier.
Of course, that's not to say that anyone else is necessarily unhappy, and there a LOTS of people, of all races, who love the stereotypical 'black ass' but more than anything else, bubble butts are striking specifically because they're non-pretentious. Someone simply has a 'nice ass', without shaking it all over or flaunting it. It's the perception of health and vitality, rather than 'will she produce a litter of kids' that draws the eye. Leers, though, are another matter entirely.
P.S. On a personal note, I come from a family that's predominately bubble-butted. My mothers has wider birthings hips, but my father, grandmother and even great grandmother had tiny round butts. I thank our Norwegian heritage. While I was never considered a 'hunk', or even got many dates when young, lots of people would stop and stare as I walked by, both males and females, so there's definitely a certain 'universal appeal' to it.