It's actually a fairly recent phenomenon in the western world even though it's been around in comic and print form for centuries. I think it started with a few developers doing incest content to be different, new, and "edgy" in an effort to grab a profitable niche a few years ago. Same with many of the consumers- it's something new, exciting, and taboo all wrapped in a package that is easy on the eyes. Like lemmings, many other devs fell over each other to "jump on the wagon" for a "piece of the pie" and now it's crowded with mostly rote, bland and boring offerings. It's gotten to the point where it's literally over-saturated in many ways.
As with all things, they ebb and flow like the tides. I've noticed that it seems to have started to lose its lure because of it being crowded. With all the activity going on, it has attracted unwanted attention. Just recently, Patreon started a crackdown on incest content and they're not the only entity doing that. Because of the censoring, it has accelerated the decline of the incest fetish.
It remains to be seen where everything is going to end up concerning those who make and buy incest games and at what level it will remain at. My questions are - where is this incest fetish going to become stable at? Because once it's stable, that will mean little to no gain/decrease in developers and consumers. Who are the survivors going to be once the dust settles? Will quality games be among them or not? Only time will tell.