Porn sells. Porn sells a lot. But people who make porn have the problem that nobody wants to be associated with it, so they constantly have to find new ways of getting paid to make porn. Gumroad was popular for a while, for instance, until they altered their TOS to forbid it and started cracking down.
Right now, Patreon is cool with porn. Especially with cartoon porn - IIRC their official stance is "no porn but we turn a blind eye to cartoon porn unless you draw kids fucking or stuff like that". So all the cartoon pornmongers are leaping onto it. When Patreon decides to be tougher on porn, they'll go elsewhere.