By being much, much, much better than the competition at the time (sourceforge) - github was faster, without ads, used superior vcs. Github also had Octopuss/Octocat as a logo :-) .
Yes, the gui was good (and it still is, or may be I am too familiar with it at this point, I do not know).
Another factor is that git was always FAST, and the most common operations (init, log, status, commit, checkout, push, pull) are not that complicated as people make them to be, so you can very easily start to use it for new local projects, and continue without installing additional plugins, unlike mercurial ... And at some point, when you wanted a colaboration hub or just a public remote repo, you just pushed your code to github.
