[antics]

People are probably downvoting you because it's totally off-topic, in addition to be a really well-worn question. This is something you can easily Google and get hundreds of answers for.

That said, the answer IMO seems to have something to do with the fact that some very important communities ready for a different VCS model, and some very good and influential developers answered that call at roughly the right time. For example, both Mercurial and Git were started in direct response to the Linux Kernel having their bitkeeper license revoked.

Github is really a killer app for git, and that's probably one reason it's gotten as popular as it has.