I find it fascinating that systems can be bootstrapped upon their own foundations.
 - http://news.ycombinator.com/item?id=3277079
 - http://git.kernel.org/?p=git/git.git;a=commit;h=e83c5163316f...
Edit: To down voters I think this question is important because it helps us understand why certain softwares are successful despite other successful competitors. I am not trying to start a flame war on which version control system is better.
Socially, it first had a big userbase because the kernel is a large enough project and the switch was made top-down as far as I remember (not everybody was forced to do it but it was much easier if you could). This means a lot of people had to get acquainted with it, and this set of people turned out to be somewhat competent at changing it to make it faster, more stable, and easier to use (up to a point). Linus's personality played an important role here, as he entertainingly made a case for why it was good, which created a certain "cool factor". Second, there was the github factor which allowed this user base to explode exponentially by making it really cheap and easy to use.
1. By being "stupid", git doesn't treat the user as stupid.
2. git allows repositories to be uploaded or transferred anywhere for others to use with a single, simple command.
3. git is efficient and fast. No one likes waiting for their computer to do a seemingly easy task -- especially software developers/hackers who know something should be O(1) complexity and could be written in X lines of code.
4. Merging/branching is easy.
1. Celebrity nerd Torvalds creates a vcs. It is not terrible.
2. Linux development moves to git.
3. Rails development moves to git.
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.
I picked up git because I know I needed to use SOMETHING for version control. Looking around a few years back I saw CVS, SVN, and the rest.
So I Google/ask around. I see that git is getting mentioned pretty regularly. I also see Hg being mentioned. I basically search for free hosting for both. Github just seemed more friendly. The UI was bigger and easier to read than bitbucket. So I just went with git.
Often it is the stupid reasons that people adopt something: it looks nicer, my friends are using it, I can pronounce the name.
Maybe less but still important IMHO is that it's far more pronounceable than e.g. SVN. That could help build the hype.
"Git" is definitely a nice name.
This is partly why I have, so far, not joined the "Git Hub".
All in all, I think a better name could have been chosen. But, most of the time, I only type it - and it IS certainly nicer to type than `svn', I'll give it that.