I think I could come up with a good hypothesis on this. First, Stefan used to be an anarcho-capitalist, while now he openly supports Trump and gets a lot of conservative viewers. There is at least one major libertarian website that has abandoned their support for Molyneux, and I suspect many former members of this site, who used to be ancaps, have abandoned him as well. His show has also deviated largely from discussions on how to have a free society and abstract philosophy, to why Trump isn't as bad as the media makes him out to be, comments on multiculturalism, and the authoritarian left.
To the extent that his fanbase has largely been replaced by alt-right and conservative viewers, its not clear that they are all that interested in philosophy, or having discussions on online forums. I have noticed on youtube comment sections that they usually leave comments such as "we need to secure a future for white people", or "women shouldn't have the right to vote", or "we need a new hitler", to various crude insults of CNN, leftists, professors, climate science being a hoax, etc. Its not exactly erudite discussion. Beyond that, most conservative viewers tend to be 50+; they have families, jobs, meetings...not much time to have philosophical discussions on forums, if that is their interest at all, which it is likely not. I'm not sure exactly where the alt-right tends to post, but I suspect it is on the more popular channels and various subforums on reddit affiliated with the alt-right.
And as for me personally, I would never have a discussion on these forums. On the very first page of this thread you can see that people bully each other pretty easily. If you actually want to have a serious discussion in philosophy, you don't insult people on a regular basis. Not surprisingly, this is exactly the behaviour Molyneux displays when people call into his show. If they make a mistake, he berates them for a few minutes, practically getting them to admit they're total idiots before continuing. If people on the forums emulate Molyneux, no one would want to have a discussion here, even if they were ancaps. You can see this in any number of videos Stefan puts up where people call into his show.