I think the answer lays in the fact that the general porno public want to see white girls getting fucked, it's that simple, even black guys like watching the white girls.

Black guys fucking white girls has always been a popular theme in porno,I think that the popularity of the black guy/white girl scenes is based in the taboo of interacial sex,young pretty blondes being penetrated by huge black dicks,you can see this scenario exploited by the likes of Jake Steed in his 'Big Black Monster Dicks'series.

Personally I prefer white girls,Asians and blacks dont really turn me on I'm affraid, most of the top Asian and black performers are half white anyway.