Just hazarding a guess, but don't discount ease of setting up and maintaining a subreddit, compared to a forum. And the ease joining and participating in a sub

Just hazarding a guess, but don't discount ease of setting up and maintaining a subreddit, compared to a forum. And the ease joining and participating in a subreddit (which is essentially zero effort if you already have a reddit account). Instead of signing up, giving out your email to another potential spam source, etc., etc..