Mixers can promote their mixers on cryptotalk.org too but they will not get any result. Compare the traffic of bitcointalk to altcointalks and cryptotalk, I think the signature campaign run on this forum will result in far superior results compared to signature campaigns run on altcoinstalk and cryptotalk.
These mixers will realize that the comparison of traffic that they'll get there and here are totally way out of comparison. They may continue their advertisement there but eventually, if they're not liking the results, it is just going to end up with the expected result. Anyway, no one's wishing the result to be bad at all and there's always a chance for them and it's their choice how they're going to keep up with the competition and their marketing.