Keep in mind this chart isn't a reflection of general language popularity. The Github founders are well known Rubyists - that alone helped influence Git & Github's popularity in the Ruby community.
Edit: No idea why this is getting downvoted. I'm just saying look at the numbers — 77,000 Python projects on GitHub vs. 53,000 public projects on BitBucket. You can disagree with other Python devs using Git, but the numbers are there.
We mirror to github for the social and discovery benefits. The UI has very little to do with it as far as I'm concerned.
Additionally, the front-page shows code up top and readme right below.
Bitbucket projects often show an empty wiki, or worse a useless "overview" that just tells when the commits to the project occurred.
The social and discovery benefits of Github are directly related to its UI (or if you prefer, call it UX since its all encompassing).