It's not that simple. Porn is a huge risk for a company, especially if it's user-generated content. You have to police access against underage kids who want to watch that content, and also have an enormous team devoted to immediately take down CSAM.

Lots of payment processors and advertisers do not want to associate themselves with porn because there are too many PR risks. Ask Pornhub how difficult it is to be a porn company on the internet, they've had lots of thorny problems. Or look at Reddit and how it's handling porn content before the IPO: they can't outright ban it or they'll lose users (e.g. Tumblr, another great example of what I'm talking about), so they are trying to hide it as much as possible.

Basically trying to monetize porn is not worth the headache. Last but not least, because there is so much free porn around. Risking so much to gain a few users that could go somewhere else in a whim is REALLY not worth it