Its because america is still a very racist country(both black & white) & how the record labels/radio stations/people are over there.
The most known example this is what happened to the detroit techno guys in the early days. The major labels wouldnt sign them becase they werent black enough, they said they cant market a bunch of black guys making furtristic music that doesnt feature singing/rapping, choreography dancing. On the other side they had black radio stations refusing to play techno because it wasnt black enough, the long of the short of it is white america wouldnt accept techno until it went to europe and came back via white european artists & black kids still dont listen to techno because its seen as 'white music'
Simliar things happened over here in the UK, ask any black person who was listening/dj'ing to acid house/techno/hardcore between 87-92 how much stick they got from other blacks for listening to it until jungle came along and added ragga samples, only then was it 'OK' to listen too.
Simliar things also happened in hip hop, the majors took no notice of rap until NWA came out and they saw how they could market is because sex and violence sells, as soon as that happened if you were a rapper and you didnt sing about bitch, gun and selling crack you had very little to no chance of making it on black radio let alone MTV(a station which a decade ago refused to play black music).
Now consider that and think if techno/hip hop artists are having these problems what chance do you have if your black and you play rock ?? what % of bad brains fans are actually black ? 99.9% of blacks have never even heard of bands like the bad brains, black kids today grow up being told if you wanna make music buy a sampler not a guitar.
Ill also add another reason also because the whole radio for black music & radio for rock music means most black kids arent even exposed to rock music