Git stepped into a source control ecosystem that was well-served (albeit contentious). People knew (or at least thought they knew) what they wanted from bk/svn/CVS/p4/rcs/sccs.

So git essentially was the "final form" that integrated all the various workflows and topped it off with a maximally-scaled use case (linux) that proved out the tool, drove innovation in integration/scripting/gadgetry, and provided a clear beacon for everyone else to adopt it. So it won.

But in 2004, if you asked around, everyone would have told you that a tool like this was coming at some point (even if they probably wouldn't have described it as very git-like!).