I think the strength of GitHub is lying in it's focus on code. And that's the point of a code hosting website, isn't it? You visit a GitHub repository... what's the first thing you see? Instead of a blank page with some numbers and a project summary you're directly looking at the core of every project: its code. The presentation of the readme file is just one more thing which speaks for GitHub. Your project summary is integrated into the actual process of development because it's a part of it.

Plus, you really get this feeling of activity while browsing projects and code on GitHub. Google Code and SourceForge seem so abandoned in comparison.