If you wanna go down that route, it's actually mostly Pornhub's fault.
Pornhub has been infamous for a really long time for hosting various forms of illegal and deeply unpleasant content, such as videos of sexual assaults and sex acts involving children. Pornhub itself was incredibly obstructive and slow about getting this content removed, because why wouldn't they be? It's in their interests to leave that stuff up for as long as possible.
The result was a massive public backlash. The New York Times reported on it, and Pornhub's parent company got sued by a group of women who had had videos of sexual assaults against them put there. The "anti-trafficking" (read: evangelical anti-sex-work) lobby got involved, and put pressure on credit card companies to do something about it. As a result of this Mastercard carried out its own investigation of Pornhub, which resulted in it, Visa and a bunch of other credit card companies cutting ties with Pornhub, meaning their cards could no longer be used there. Pornhub responded by removing all content by non-verified users and overhauling its (admittedly really terrible) review system, but the credit card companies didn't back down. Instead, Mastercard is moving to change their policies on adult content so that anyone appearing in that content must have their age, identity and consent verified by the banks which process card payments, which means that any bank allowing payment to sites that host user generated adult content will pretty much automatically be in violation.
That's why Onlyfans wants to ban adult content. They're frightened of ending up like pornhub and being cut off by credit card companies. They're a pretty small fish in the grand scheme of things, and weren't really a part of the conversation before, they're just pre-emptively panicking and making bad decisions as a result, but bad (or malicious) decisions are pretty much the name of the game here.