After skim reading I couldn't see it mentioned, but when SourceForge started bundling malware[0][1] into the software they hosted, it was their death toll.
As my memory recalls it, that triggered an exodus to Google Code, and whilst GH was gaining traction it was somewhat in their shadow. When Google announced they were going to kill Code that was the blessing for GH.
[0]: https://neverworkintheory.org/2022/04/21/decline-of-sourcefo...
[1]: https://news.ycombinator.com/item?id=31110206
As my memory recalls it, that triggered an exodus to Google Code, and whilst GH was gaining traction it was somewhat in their shadow. When Google announced they were going to kill Code that was the blessing for GH.