[jfaucett]

I can think of a couple reasons.

1. Github got First Mover momentum

2. Git preference over mercurial back when Bitbucket burst on the scene.

3. Github has many users which means its easier if you want to reach a large audience with your FOSS project.

4. Due to #3 large companies hosting all their big projects on GH, leads to a self perpetuating cycle.