They use GitHub to the max so I'm guessing they actually like GitHub. I've heard people who like GitHub complain about other code hosting sites not having a source tree on the project website. Never heard an avid GitHubber complain about the source tree on the project website. So I'm guessing that it suits their aesthetic just fine.
I get a bit put-off when the first thing I see on a project website is a source tree, unless that's what I was expecting.