I think that there is a point on those stories that's about the whole conquering process. Don't know if this is the best term, because I'm not native english. But the process is: you are single, you meet a single girl (boy perspective), you say something nice, start to know her, give some presents, many steps later you two start to have warm feelings and then sex. Regular sex in a general rule.
Sometimes people get tired of this and start to look for alternative paths, to cheat the rules, jump some steps, make something from a complete non-sexual contest direct to the some action. The classics were the professional relations, so you have the pizza delivery guy, the plumber, teacher, doctors and so on. Porn industry used it a lot, abused in fact, until the point they become almost boring.
The next natural frontier was explore something more intimate, more inviolated, that would be family relations. Imagine that you have a hot mature lady as your neighbor and she have a male son. Can you imagine something more trickster then if was physically possible to embody in her son? Spying she on the shower? Getting access to every private room? Touching her pretending to be inocent? Provoking her until the point that she would make something really forbidden? That would be completely cheat the conquering process.
This theme still has more muscles because of many code of laws laws that prohibit it, less creators could explore without consequences, specially in the past decades. Imagine a small movie company rolling a film with the theme, the police come and arrest everyone, causing a great money losses. Games were even expensive and hard to make, put all that effort in something that could lead you to jail, shouldn't be logical.
Then big sex movies portals like xvideos, pornhub, redtube... appeared allowing cheap and less risk productions. You start to see several short movies about it. Years later, "easy to use" 3D engines like Daz and a viable way to make money before getting the product ready, like Patreon, turn everything cheaper and profoundly less riskier. So the theme was attacked with fury.
Patreon decision to ban incest theme since last year, maybe will great fuel for a new wave in the next years as soon as a new crowfunding platform come up allowing everything.
One thing that I believe is that this kind of theme being explored on movies and games, doesn't mean that incest will start to happen all around the world. The base point is that you don't see your mother in the role, is always someone else's mother. Something similar as you to see yourself as a plumber that was going to fix a sink and a hot housewife came at the door. Well that was my theory
