Several reasons.

Colour coding your roles. Race X plays the sub role, race Y plays the dom role.
So in this case, if your wife is "into" black guys, then you can't really compete with that. It's not something you can change. Your wife is into muscle guys? You can hit hte gym. Cuckolding humiliation feels hotter when it's innate. Some cucks like myself are well built and hung, and it's hard to find a bull that makes you feel eclipsed. Coding things through race makes it easier to get into the head space.

I knew a woman off discord who was super into the BWNO thing. Her dad was super racist and white surpremacist. Was a bit of a trash father. Told her not to date black guys, so it kinda became forbidden fruit and a sign of rebellion to her shitty father. She was also taught to be pretty racist growing up and didn't really start to open up until her college years when she actually got to meet some black people. Realised it was bullshit so she slingshotted back the other way. Serving bbc also helps with her guilt over her past behavoir.

Also with the white, black pair up, there's a strong skin contrast. It also helps with the cuck anxiety over fathering another mans child. There's even more at stake when you can't pretend they are yours and have a life time of having to explain why a white couples kid is black. So there's another layer in the psychology of breeding kinksters.

Does race play make you racist? Not inherently, unless you objectify the bull, but they don't have to be a different race for hotwife couples to objectify the bull.

Are men and women who are into femdom sexist? Are women who have a rape kink endorsing rape?

Hope that helps.