The major credit card transaction institutions decide what is allowed or not for commerce. That's Visa and Mastercard primarily.
So, they are okay with violence, murder and porn (for the most part), but incest hits a bit too close to home so they don't want to be associated with that.
Funny that you mention it since today subscribestar changed TOS due to payment processors to ban incest, rape, gore and etc in fictional media
back to og topic.
In general, it's an interesting topic, and I think there's way too many different spheres of influence that would be impossible to count.
Firstly, the way I see it, the reason is a general public exposure to a mass amount of people that aren't simply used to it, or accepting to it, this eventually leads to karens (male karens included) complaining, which leads to a bigger outcry within the news circles and beyond, not to mention mass hysteria. Companies not wanting to deal with it just go with appeasement strategy. And same thing were true for games involving violence in the past.
For example the og mortal combat (And not the arcade version either, but the later console one) is the reason why we have a ratings board (GTA series also has it's fair share of controversies, as well as many others). Ironically when it came to movies nobody had issues over violence there, but god forbid someone showing a kiss scene or a romance scene, still remember how my parents acted between the two situations. These days it's not as bad when it comes to violence in fiction, but you can see that these same people moved on to anime, porn, and anything else instead.
In the past, most people didn't know what anime was, what lolies were, or hentai, now it's within a much bigger public sphere, and outcries about it are ever increasing, thus many companies try to stay away from it and the karen governments acting up more than they should.
Now about those companies, most of them hire the people that complain to show where the issues within the companies (including governments) are in order to save face, and avoid bad PR in the future. Which lead to incentivizing these kinds of people even more, and giving them power that they should never have, which lead to inner decisions that cater to such individuals, and here we are now.