I would think the only reason is the chargeback/fraud rates are too high in this type of business, as well as not wanting to be involved in a business where there may be a high chance of illegal content.
https://www.bloomberg.com/news/articles/2021-04-14/mastercar...
>The banks will now have to ensure that sellers require “clear, unambiguous and documented consent” in adult content, the payments network said in a blog post Wednesday. The firms will also be required to ensure websites document the age and verify the identity of anyone depicted in pictures and videos as well as those uploading the content.
>“The banks that connect merchants to our network will need to certify that the seller of adult content has effective controls in place to monitor, block and, where necessary, take down all illegal content,” John Verdeschi, Mastercard’s senior vice president of customer engagement and performance, said in the post.
Allowing merchants to sell pornography is probably not worth the hassle for the card networks.