Weird they wouldn't switch providers for specific use cases. Does Stripe require to be exclusive provider for site?
The other providers have similar rules.
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.