Incest porn, according to
this article from 2015, seems to be a matter of oneupsmanship. Which makes sense, given how the internet pushes people into harder and harder things to increase viewership and retain viewers.
It's similar to how YouTube starts off showing you generic videos, and gradually learns what you like. If you watch video games, it pushes more video games. If you watch politics, you get more politics. If you watch music videos, you get more music videos or music theory. But they know people get bored easily, so they keep trying to push you into longer and more complicated videos. If you watch a tutorial on how to use a radial arm saw, you get videos about woodworking theory and project builds. Makes sense, right? They want you watching longer and more often so they can show more ads.
But if they can't hook you on something, you'll just watch that one video on how to use the thing, or listen to the one song, and then you go do something else. Suboptimal, from an "eyes on screen, watching ads" perspective.
So they recommend clickbaity titles that will provoke an emotional reaction, good or bad. Most folks will click to see what the Top Ten Music Videos That Made Us Cry this year were, or Twenty Baking FAILS That Have To Be Seen To Be Believed. Humans really like bulleted lists, especially if the thumbnail shows a girl accidentally spilling her tits out on camera. If that doesn't work. they try to hook you with something political. The old thrill of watching a guy be amazing at Mario 64 isn't there anymore, now that we can watch Dr. Video Games own the libs/conservatives with facts and logic and/or explain how playing Sonic all day is really the same as volunteering at a soup kitchen. You're not just a passive viewer anymore, you're part of an in-group who "knows" and "understands". And to remain in the group, you need to keep watching.
It's similar with porn. For those of you old enough, think back to the first porn you ever saw: some airbrushed nudes or Playboy centerfolds probably, eh? And then just a woman with her tits out doesn't do it; you need to see her cunt and ass as well. Then, she needs to be doing something to herself. Then, she needs to be getting fucked in progressively more taboo holes. She needs to be fucked by multiple guys at once. Keep going and stacking more and more extreme things, and you reach where we are now, where the woman needs to be both your sister and brother and you at the same time. I'm just waiting for simulated snuff porn or something to be the next big thing, because the kids who started out on interracial gangbang incest need something harder.
See also, opioid addiction. You keep needing more and more to get less than the effect only a little used to give you.