GitHub won well before because git was superior to existing open source SCM solutions and GitHub created Pull Requests which made forking and contributing code

GitHub won well before because git was superior to existing open source SCM solutions and GitHub created Pull Requests which made forking and contributing code back from your fork trivial. The only real question at the time was whether Git or Mercurial (or for a brief period of time Bazaar if I remember correctly). While Mercurial itself was clearly superior to Git, it had no equivalent to GitHub. If GitHub had chosen to build their workflows on Mercurial instead we would likely be talking about HgHub right now instead.