GitHub offered a better experience than the existing offerings. They then scaled massively to the point where it was expensive to run without a lot of good options for monetization. Thankfully for GitHub they got bought by Microsoft and not say Verizon or GameStop (which owns the corpse of SourceForge for unfathomable reasons).

GitHub could have easily enshittified in an effort to make money had they not been bought by someone with deep pockets.