Yeah, I think it’s this. Scat porn, for comparison, turns off most people so having a bunch of it up on a main page is going to hurt traffic. I’m guessing there must have been some legal issue that turned all the incest porn into step incest porn, but regardless most people probably don’t care if a scene starts with someone calling the other person “step brother” or something. Some people probably actually prefer that. So the benefits outweigh the negatives for the sites.