I agree with that generally. Though a big part of that reason I think is most mainstream special interest communities established their main forum sites/communities long prior to the existence of reddit and "web 2.0" social media.

I would say that newer communities tend to coalesce around reddit / twitter / discord first now, because it's the path of least resistance. Communities focused on newer technologies / fandoms / arts from only the last decade or so seem much less likely to have dedicated high-traffic old-school forums now.