I think it has to do with the perspective. The man in a cuckold relationship knows his wife is having extramarital sexual relationships. I think it's some sort of a fetish.
Because Porn is aimed at men. If you use the broad definition where Cuckquean is really just a woman sharing her man with another woman, then there's tons of that, naturally. But if you're looking at the more strict definition where there's the degradation in relation to the other woman I don't think it's that appealing to men.