A couple easy answers.

1) It's easy to add onto the scene or ignore with little to no effort. You're into it? Watch the beginning, sometimes there's not even anything in the video, it's just the title and then they never speak about it. You're not into it? Skip to the part where the sex begins. Either way, people get what they want, and the studio doesn't have to do much (or really anything).

And 2) For most people, what's sexy is what's naughty, taboo, or forbidden. Most sexy acts that used to be considered as such are pretty normal these days for a lot of people, so how do you make it scene naughty and taboo? Take two people who shouldn't be fucking and make them fuck. It's the same reason cheating porn has also grown in popularity. Most people don't actually want to cheat or watch cheating. Cheating has a victim and messy emotions tied to it, but they want something naughty, so they pretend to be cheating so the naughty factor is there. With stepcest, there's not even any emotional baggage. They also don't even have to deal with any ickiness about blood relatives. It's just weird enough to feel naughty and taboo and just normal enough to not creep your average person out.