Payment processors view this as a risk/reward scenario. NSFW content historically has very high chargeback rates. When your overall processing amount is low, the risk is low. As the amount goes up and your business gets more successful, the risk increases as your new customers are more likely to be fickle.

This is a few years out of date but when I investigated what it would take to legally accept credit cards for NSFW content, the processors all wanted a very large bond to hold as protection against chargebacks. I imagine once your daily transaction volume grossly exceeds reasonable bond levels, the processors get pretty flighty.

IIRC there's basically one payment processor, whose name I can't recall, that ~100% of NSFW sites use. And their fees are very high, given the risk of processing those payments.