I was listening to a podcast somewhere that interviewed a performer (this was years ago) and it was said that when incest porn was on the rise it was because there were one or two really rich dudes with the fetish who were bankrolling a ton of scenes like that and they, of course, were releasing them for mass consumption in order not to just be blowing money on the shit they were directly into. It proved popular enough that it kind of took off. I think the second part is where Christman's analysis comes in.