Some amount of adult comment is CSAM, or otherwise broadly disfavored. Some companies (Pornhub, OnlyFans) are willing to specialize in discriminating between “regular” adult content and the objectionable stuff, and they have payment processors similarly willing to specialize.
Some of that specialization involves being willing to take on political exposure. Mainstream payment processors are unusually exposed to risks like “being dragged in front of Congress” — there are a lot of reasons a politician might want to put pressure on a general financial infrastructure provider. So reducing obvious ways to get embarrassingly dragged in front of Congress is rational.