It was a trend that worked for a tiny amount of people then others started doing it too.
The porn industry is just like any other marketing structure; people put products out, see how the consumer reacts to it, then capitalizes on it.
Take Belle Delphine for example.
She became a millionaire by popularizing a bunch of weird, erotic fantasy adult content, one of which was the infamous "ahegao face" shit that she integrated from hentai. Then, for the next 2 years straight, EVERY pale, super cuter, petite adult content creator started making tons of content involving that stupid ahegao shit in a desperate attempt to pull the same kinda numbers as Belle.
And the shocking part is that it actually worked for a number of them.
Even to this day you will some creators still chasing after that method.
My point is, I think the same shit happened with this "incest boom".
And like any other "product" on the market, the more people seek after it, the more content creators will continue to milking it while it's hot.
Things will remain that way until new trends pop up and people start losing interest.