In part, I would guess, because if they did that, there would be a much higher likelihood that payment processors would decide to blacklist the Red version of the service—something they can't really do with just the adult streams on Twitch as it stands.

I would further guess that this would only be one aspect of the broader marginalization of the Red version of the service—they wouldn't be promoting it openly on the main Twitch, because that would antagonize the "think-of-the-children" types, and it would no longer benefit from cross-recommendations and from people watching a Call of Duty stream until 11PM, and then noticing in the sidebar that their favorite adult streamer has come online and switching over to them. They would probably even suffer from the one quasi-legitimate excuse the payment processors have for banning adult sites: much higher rates of chargebacks, as people with overly judgmental and insufficiently communicative significant others try to hide them on their credit card statements (or any of a bunch of other reasons).

It would be great if we could collectively treat sex work as work, just like any other form of selling our bodies and minds to support ourselves—and, indeed, for the time being that is effectively what Twitch is doing, and in doing so benefits both themselves and these streamers, who otherwise would have to use more stigmatized platforms with much less reach.