Could also be that those are the only porns with even a half assed attempt at a plot instead of just opening with a ground and pound

Personally I think the rise in incest porn has to do with the rise in isolationism. Lots of people, young men especially, are going out less and less and having more of their social interactions online. As a consequence of this, for a number of these men, the vast majority of the real life female interactions they get are from women in their own homes. And biology has a way of adapting, so I think a lot these men are getting confusing feelings about people in their own homes due largely just to lack of outside exposure to women.

As the other comment implies, that's just the easiest plot to shove porn into. I don't think that many people are watching it for the plot, it just happens to be there. Like, people aren't fantasizing about becoming a plumber to fuck women instead of being paid, for example. It's just a lazy plot because they need to give some reason for the scene.