The chargeback rate on online porn is huge. As a result, where a traditional payment processor may be 1.3–3.5% depending on the business and the assessed risk, for a high-risk business, the rate can be much much higher. While Onlyfans keeps 20%, I'm guessing that they'd be badly hurt if they had to give half that to their payment processor. I'm sure they've looked at their numbers and they find the non-porn providers are more profitable to them, especially if it means they can lower their processing fees. Booting the porn providers will likely also make it easier to recruit more non-porn providers.