It's a messy subject and we've tried many things in the past but it all ends up washing out and having the same cycle repeat itself. Like encouraging folks to post in the megathreads, it works for a while but then new creators show up and make their own threads anyway.
It's evident enough how many topics I have to close that don't follow the forum guidelines (mostly talking about webtoon or soliciting sales in one form or another) that new users here don't actually read the guidelines in detail. So adding any new rules in regards to promotion would mostly just go ignored.
And as vibrantfox mentioned, these forums run on a separate engine that Tapas didn't build from the ground up, so they are limited in what they can implement.