This whole debate has been done to death here, as stated.
There are various 'reasons', and of those who do refuse to see black guys, WGs will feel one, or a number of those reasons apply to them.
Could be basic racism.
Some WGs are 'hobbyists' and don't fancy black guys IRL.
Her pimp might be black and he might not like it.
Her BF might be black and he doesn't like it.
The WG might have experienced previous intimidating/violent episodes which she sees as more frequent with black guys.
She might perceive black guys to be more likely to have a big dick, leading to more downtime and loss of earnings.
She might think black guys like to be too rough.
Over time the average WG will experience most of the above, from various guys, from various racial backgrounds. In her head she'll form an overall impression of how various races are. Whether that impression is statistically accurate is a matter for her. IMO women overall tend not to be as rational/analytical as men (but excel in other areas where men fail, choosing curtains etc
), and for women who go on the game, you can probably increase the likelihood of that, which in a way is understandable when they're meeting strange guys for paid sex, normally without any meaningful security.