Earlier quoted context omitted.
Two reasons. One is the higher incidence of chargebacks. You don't have to ask if something is naughty -- just look for the chargebacks and it'll tell you. What happens is someone charges a thing, their partner sees it on the CC statement and asks about it, the first person is ashamed to admit it so they act shocked and insist it MUST be fraud because THEY would never charge something like that, and to keep up appear…
There is one more: legal compliance. While there is no law on the books that says banks can't do business with OnlyFans, there are laws ( BSA comes to mind ) that in effect create incentives for banks to 'derisk' problematic customers. Some get branded high risk based on internal and industry standards.
Also, the type of people who are typically sitting in the meetings who would have to sign off on changes to these long-held banking “truisisms” haven’t given it a second’s thought and they really don’t care to re-examine their beliefs anyway.