I’m guessing there must have been some legal issue that turned all the incest porn into step incest porn
I believe in some jurisdictions it’s illegal to depict nuclear incest in porn, even though it’s fictional.
Also I would speculate that the actual fetish comes from the fact that a decent amount of people would have had a step-sibling added to their family during their adolescence, which could have influenced their fantasies and taboos. If you’re a 13 year old boy and suddenly there is a 14 year old girl living in your house that you aren’t blood-related to, it could easily lead to some weird fantasies popping into your head.