Why Is Step-Incest Porn So Insanely Popular Right Now?

I think it is due to it being a easy/lazy setup for writing the story. First locational and setup, you can use a house or apartment easily and explain why these two people are in proximity of each other in this setting. Second the taboo-ness gives a reason why these attractive people have not acted on their basic impulses yet, thus creating conflict (storywise) that they have to resolve, que the banging. It is taboo-ish socially wise but not legally wise, like many other genres.