The writer's answer is that it's pressure from their racist boyfriends and husbands. But of course! I mean, we know in advance that it's the fault of white males, we just have to figure *how* it's their fault.
More realistically, performers who don't do blacks are probably concerned about getting their pussies stretched out of shape, and less attractive to many of their viewers. An ******* we know doesn't take black clients for the same reason, and she says that's fairly common. A tight pussy is prime business asset in her profession, so can you blame her?