In reply to Rick Dees (@r1ckd33zy).

The short answer is because WordPress predates Git by about a year. We've been using SVN for longer than Git has actually existed. Switching to git would take a fair amount of work, for not a whole lot gain.

That said, work is being done on things like git syncing and other means of making WordPress and associated bits available for those who prefer to use Git or SVN. That work continues.