The problem is also observed by the fact that "black" women get a special label. "Normal" sites offer "normal" white girls.
In response to what people posted earlier in this thread, I do think it is a matter of supply and demand. The thing is, that does explain how it can happen but not why it happens. It is not an explanation in the full sense.
My guess would be that for some reason "white" girls are more favored by the entire (male) market. Assuming that the market is composed of man of all ethnicities that would mean that the "white" girl is the ideal girl.
Perhaps power relations can be a part of the answer, after all just decades ago people spoke of the "white man's burden": the white people were the most progressed technologically and economically and therefore the most powerful. (the "burden" was that the white poeple had the duty to help the others progress) Because they were the most powerful they were able to influence the system a lot more then the others.
In the current age, this especially refers to American hegemony.
Important: I do not agree with the White man's burden ideas or anything of that sort, above all else I'm a european first, and a cosmopolitan second.
But that's just what I think, powers shapes perceptions. And competition leads to imitation (of the "best" practice).