I never worked in porn but I did work with payment and clearance systems from carrier companies a long time ago, think billing customers for SMS messages to specific numbers and then getting clearance for the carrier for the money. The risk of fraud or any kind of weird issue is stupidly higher for porn. Either simple "customer doesn't have the money" fraud or "the police knocks on your door because you got money from X customer" fraud. So maybe you need more people or better systems to deal with it while still being profitable. CCBill charges are huge when compared to other credit card processors.
It's easy to reduce costs and complexity until the easiest solution to keep or improve current margins is don't do porn.