The reason behind banning adult materials has to do with Puritanism and with the high rates of refunds on adult websites.
Momentum. They are the big games in town because so many people use them, so many people use them because they are the big games in town. There was a time for both when they didn't suck as much as they do now, at least relative to what other options existed.
Yet, the payment processors will all reliably treat anything NSFW equally by suppressing it as much as they can. From banning individuals who dare do transactions they don't approve of to directly pressuring websites that might tolerate NSFW content by threatening to take away their only means of making money. If they only cared about refunds and profitability, they wouldn't ban individual artists - because the fact how these artists often manage to stay undetected for years suggests that many of their customers aren't the kind to start complaining.
It's quite fascinating how this is the one area where the companies are willing to "self-regulate". They don't process sales of illicit drugs because the governments above them said no and put in extensive guardrails to make these illegal uses as difficult as reasonably possible. Yet, despite most first-world governments not taking issue with adult content at large (for now), the payment processors will act on their own and diligently turn away any potential revenue they could be collecting.