That and the fact that people are more likely to excel in what they already know. As far as black people go, the majority in Britain live in the biggest cities, and usually in areas where they'll be most exposed to 'urban' music (by which I mean hip-hop, R'n'B, Drum'n'Bass and also a bit of funk, blues and soul etc.). Most black kiRAB' parents don't listen to much rock music of any kind, so its unlikely that their kiRAB will develop a taste for this loud, guitar and drum driven style. If they don't develop the taste for it then they're not likely to want to perform it and if they turn out to have some kind of musical talent, they'll choose a different style in which to express this talent.
A similar thing can be seen in America too, although I don't have first-hand experience of the ethnic layout there...
Also, as unfortunate as it may be, black people are still more likely to occupy the lower rungs of the social strata (more likely to be poor and/or working-class) and rock music requires decent instruments, speakers and sometimes rehearsal room rental. These cost money, which is more of an issue if you have less of it. Styles like hip-hop, on the other hand, don't tend to require the same financial investment (ok, this argument isn't as strong...)
