You're right about advertising, but they also are a legal target. Any platform that has sex related content on it will eventually feel pressure from their country's government. It just takes one incident and
I reckon platforms see them as a burden due to the volume of regulation that they inevitably become saddled with, or face penalties. Hence why tumblr rolled the dice on removing it all, Reddit is restricting ease of access, and Pornhub purged most of its content.
I'm interested to see how the Fediverse will handle it when it eventually comes this way.