Finance is the single most heavily regulated sector of the economy. Almost certainly someone in government coerced them to stop taking these payments, exactly the same as has been done to unbank other disfavored industries.
This has resulted in payment processor execs historically being very prejudiced about against any site that provides that sort of content.
Combine this with the dual facts that no one of good standing is very motivated to stick their neck out to defend porn publicly, while many people define their politics by being very publicly against it, and you get a system that routinely discriminates against sex workers.
[1] Historically, this was from angry spouses/parents seeing it on the CC bill and the person who ordered it lying that "someone else must have stolen the card and ordered it"; nowadays actual identity fraud is so common that it's a real concern
It feels like this is one of those things that card networks say because they know folks who hear it won't think about it more than a few seconds to realize how much sense it doesn't make.
You imagine that the guns of government regulation will always be in your favor, and are totally surprised when they are pointed at your head instead.