I really think the answer has to do with Github's take on what a "public" view looks like. They focus on the actual files (which is the primary use case of a repo host) and then they focus on the README file (the WHAT), and finally focuses on the contributors (the WHO). It's not as pretty as Gitorious, but it is functional and clear and it gives me a clear indication of what I'm getting out of the site and why I should care.

The ability to link to the Github 'public page' also helps make it go viral -- I never actually ended up in Github's homepage until today, I always ended up there because of a link I followed for an open source project, and then was upsold to sign up and set up a private repository.

On the other hand, Gitorious doesn't make it obviously easy to find the public repositories, makes their front page look like yet another project management site and even when you click on one of the obscure links for a public repo, you're shown a bunch of Activity feeds which doesn't mean anything to the typical visitor.

I think the hardest thing to do with WebApps is to show the user what a "humming" version of the app looks like, how a productive user of the site can use the WebApp. Github does this excellently while Gitorious does not.