I'm pretty sure the only thing that anyone cares about when they're deciding what porn to watch is:

* Who's in it

* What they're doing in it

* Is there annoying fucking music playing in the background

But the industry probably made a really good choice of these things that happened to appeal to a lot of people in many of the movie's scenes, that *just happened to be an incest thing*. Then some analytics person told them it was getting a lot of views/purchases. 

Given the analytics and confirmation bias, they started putting their most popular stars and directors into this thing and because suddenly the most popular stars and sex acts were in those videos everyone started watching them.

It was successful (for other reasons) so they just didn't stop doing it because they think it's the reason it's working. Most people are annoyed by the fact that it's incest porn, but not enough so that they aren't just going to skip past all the dialogue to get to what they want to see.