There are very few payment processors that will accept adult content, and those that do exist (e.g., CCBill) are both expensive - on the order of 10x the fees -

There are very few payment processors that will accept adult content, and those that do exist (e.g., CCBill) are both expensive - on the order of 10x the fees - and also a damn nightmare to work with. I can very easily imagine a scenario where Patreon looked at their options and decided it wasn't worth it.