This issue is undoubtedly a complex matter, and there is no one, universal answer to this question. Browsing opinions on the internet you can find a variety of reasons.
Probably, one of the simple answers is a prejudice against people of other cultures. Sometimes, the problem is caused by the girl's prior experiences. If a girl has gone through several unpleasant meetings with black men, she will naturally avoid them in the future. There is a stereotype black men are more persistent when rejected by a girl. It is, of course, a big generalization. However, it is just how do stereotypes are created. Prejudices of this kind may become a fundamental matter for some Escorts.
There is no clear solution to this; however, some girls tend to do some kind of "racial profiling". They just instantly assume that a young, black man can possibly become more dangerous than an older, white man. Of course, irrespectively of skin colour, each of them can turn out to be dangerous, but the prior experiences and stereotypes are likely to arise among girls.
Sometimes (probably for some humorous purposes) you can find some opinions about penis size. Due to the general opinion, black men usually are "better equipped". Bigger penis equals more intense pressure for vagina; therefore, some Escorts prefer to have sex with men with smaller measurements.
In order to avoid any racial discrimination, you should use a professional Escort Agency that will take care of your safety and can offer you a girl without unreasonable prejudices.