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).
So, the other platforms don't also want to allow them for advertising space because that could give them an eye from the government that tracks down dirty money and uses them for it. It's also possible that the next place that they're going to advertise might get some caution of giving them a notice to disallow them to advertise. But let's see, just like the other conventional websites and platforms, they're just doing safety precautions and filtering the ads that they might allow to pop on them that won't have any troubles caught by the regulators.