I assume stupidity is to blame for SourceForge's current unpopularity. If they hadn't started bundling spyware with downloads and responded to GitHub's better features, their huge repertoire of legacy projects would have kept them relevant.
That and GitHub's UI was so much cleaner and professional looking. Even without the bundleware fiasco, the website was dated, and a maze to navigate. GitHub looked like the cool new kid on the block. I'm sure it would have been popular either way.
CLICK THIS HUGE BUTTON to DOWNLOAD (some advertisment on the download page)
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(tiny download link)
That's exactly why I'm skeptical of clicking any sourceforge link. I don't understand projects that haven't left.