I'd assume the reason is that those roles assert dominance and hence why they are fetishised.
In a typical hentai, dominant individuals tend to imagine themselves being on the top and dominant position whereas submissive individuals tend to imagine themselves being in the bottom and submissive position.
This can be applied to rape hentai where people who read the story with a kink for rape would find it relatively sexually arousing imagining themselves being in the shoes of the sexual harasser or in the shoes of the victim.

Anybody else notice that hentai manhwa/manhua always have an extremely fucking gross, ugly/below average, pervy, boring protag that somehow attract a bunch of really hot women? like, at least in manga the guy is usually just sorta boring and average looking but manhwa/manhua protags are LITERAL fucking scum.