First, fuck reality. In a shooter, Grand Theft Auto, etc… I’m God Mode and can do horrific crimes. IRL, I’d have had my ass kicked by law enforcement right from the start. So, reality in porn is a hard pass for me re: Incest, harem, meet and fuck, etc…
Second, as many have said, something forbidden is attractive, like many fetishes.
In my opinion, incest porn has a big following because so because some (many?) porn mediums (videos, games, books) actively remove it, either by choice, by Credit Card / Bank enforcement, or actual local laws. In the 80s, where I lived, it was just a porn type, like any other (anal, lesbian, etc…).
Finally, unlike some other forms of porn, with games, it’s easy enough to restore the content with a simple word substitution:
Landlady -> Mom
Tenant -> Sister
Ward -> Daughter
Etc…
And there’s nothing much the hosts of said game can do against a simple patch / URM mod that turns:
“We can’t do this. It’s forbidden. It’s wrong. I’m your landlady!”
To
“We can’t do this. It’s forbidden. It’s wrong. I’m your Mom!”
It’ll be interesting to see if (when?) CC companies and legislations don’t give a damn anymore and incest is back to “acceptable” porn if its popularity then dies down.