SourceForge itself is free software. I think that's one possible reason to use it rather than GitHub or else.
The other possible is that SourceForge provide mailing list server and other service for projects. Anyway, you won't move a project around unless you have some strong motivations.
Is SourceForge really open source? Guess I must have missed that news. Regardless, Gitlab would still be a better choice in terms of UX and is also open source.
I agree that sourceforge.net as a service might not be my favourite, but Allura the forge-software seems pretty good. I think the only thing that comes close is the stack from Canonical -- but Allura seems more viable to self-host (and scale up when needed -- rather than "only a vm" or "full cluster"). It also seems less "overly opinionated", and still includes sensible defaults (contrast with getting up and running with Trac).
YMMV, but it doesn't look half-bad. Right now I don't need anything more than what you can easily get with either a separate wiki+mercurial+mailing-list, "just" trac or fossil -- but for something in-between "too big to host internally" and "too complex for Trac to be easy" -- it looks rather interesting. I'd love to hear any war stories.
GitLab developer here; it does! Where GitHub only allows images to be dragged into issue and comment description fields, GitLab allows this for any type of file.
