You are asserting that cuckolding is more prevalent than cuckqueaning without any data to support that. I happen to agree with you based on my own anecdotal experience. So I hope you'll forgive if I continue without trying to look up numbers.

I think the answer is racism. Black men are hypersexualized. Smarter people than me have written books on the subject, but the gist is that there's some overlap in America and related cultures between traits that are considered "black vs white" and traits that are considered "masculine vs feminine". For example, look at how transphobia, misogyny, and racism all combine into the conspiracy that Michelle Obama is trans. For white women, having pale skin has often been seen as attractive and feminine (although tanning complicates that as fashion trends change).

A lot of men are suppressing both their own homosexuality and their insecurity about their masculinity. These are two separate ideas: there is still a good amount of cuckolding porn without a "black bull", and there is a good amount of "BBC" porn without cuckolding. But the overlap is pretty significant, and that's where we see the trope the "black bull".

And in case it needs to be said... I don't mean to kink shame but i'm not sure either of these are healthy ones. Especially the racial part, because that causes issues with expectations for black men. This also has a tendency to portray women as prizes: valuable objects which the cuckold has lost, who lack their own agency, and are much more often white. This is also probably stoking the fears of "white replacement theory" that are leading Americans to fascism.

Cuckolding without a racial component is less problematic IMO. It could be seen as stepping a toe into bi-curiosity, or perhaps even a bit of asexuality.

As for why cuckqueaning is less prevalent... I think it it partially categorization, but also partially what we expect to see from porn. A woman dominating a submissive couple is probably not going to use the word "cuckquean", though it could. Plenty of FMF 3some porn where they all just have sex with each other and there is no "cuckquean" being forced to watch from the sideline.