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 the league. 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.
There are many platforms that mixers can use to conduct promotional campaigns for their services, so I tried to find an explanation as to why they prefer the forums and insist on them, even though most if not all of them cannot be compared to the BitcoinTalk forum, and it can be said that they fail due to the lack of traffic on them.
The only logical explanation that I found is that all the rest of the sites refuse to place advertising banners for Mixers, knowing that as far as I know, there are no laws or legal conditions that prevent them from doing so (at least until now).
You're right, there are many mixers that are trusted and reputable, they are not the ones involved in these evil deeds, people are, mixers only provide for privacy, there are many other platforms they can use to make promotion of what they are offering, the only difference is in them acheiving their main target audience like the one they gets from bitcointalk through traffic generated for them when they are making their promotions.