I think ultimately Sourceforge fell from grace because it wasn't "fun", whatever that may mean; it was lacking the social features, poor UX, etc. Many things that make it boring or "not fun".

I've looked at some tools used by Google - mailing lists, Gerrit, etc - none of those look fun. For a casual like me, fun is important.

I don't do much with open source because I don't have the headspace, willpower or persistence to contribute much so the discussion is a bit wasted on me, but I've used both Github, Gitlab and Stash (Atlassian) for work projects; the feedback loop that tools like this give is super important to me.