The article does a good job explaining why GitHub won the developer mindshare, but there was also end user mindshare.
Unlike GitHub, where the source code is front and center, SourceForge always prioritized showing a project introduction page with screenshots and a big download button for the end user. SourceForge is where non-developers went to download cool freeware. It was like F-Droid for Windows. It was meant to be the official website for the projects it hosted, which is why it didn't host forks.
But the market for end users who download executables from random websites has been shrinking rapidly for two decades. Nowadays, either you're a developer and care about the source code, or you're an end user and just want to install that app from your favorite app store. Not to mention that most active open-source projects these days are made for other developers and not end users, so there's no point hosting them on a platform designed for end users.
Unlike GitHub, where the source code is front and center, SourceForge always prioritized showing a project introduction page with screenshots and a big download button for the end user. SourceForge is where non-developers went to download cool freeware. It was like F-Droid for Windows. It was meant to be the official website for the projects it hosted, which is why it didn't host forks.
But the market for end users who download executables from random websites has been shrinking rapidly for two decades. Nowadays, either you're a developer and care about the source code, or you're an end user and just want to install that app from your favorite app store. Not to mention that most active open-source projects these days are made for other developers and not end users, so there's no point hosting them on a platform designed for end users.