It's bigger than him. GitHub built their SaaS on top of it. Had they chosen subversion or fossil, history would be very different.
People use git because everybody else uses git and it started because GitHub provides easy code sharing and a better UI than the alternative — SourceForge and mailing lists.
> The first open-source DVCS systems included Arch, Monotone, and Darcs. However, open source DVCSs were never very popular until the release of Git and Mercurial.
People use git because everybody else uses git and it started because GitHub provides easy code sharing and a better UI than the alternative — SourceForge and mailing lists.