Reasons that I’ve read (this topic has been covered before) include concern for them being more likely to be suspected as pimps, a perception of them being less likely to pay, OR the fact that the ad stipulates this restriction may be posted by a handler who happens to be a black male who perceives other black males as potential handlers/pimps looking to lure his talent away to the competition. There is also the fact that it’s personal choice.
Again, this is just what I’ve read. I’m not a black male but I have read up on this topic out of curiosity.