It's because Visa got sued, lost, and it was found out they knowingly processed payments for illegal adult content, so they basically avoid the sector entirely now. Economist had an article about it maybe two years ago and came to much the same conclusion you did. IIRC, the failure in their mind was government not stepping in to make a law so things are less ambiguous in the future. Now payment processing cos get to gate keep people's speech, which means everything is basically a civil suit away from getting blacklisted.