Because Go is at least as fast as PyPy or Cython, have a similar coding feeling to Python, have language level concurrency support, and has a clear/consistent development roadmap moving forward.
PyPy is not really a good example here. It is still highly experimental, takes long time to compile, it's not compatible with quite a few libraries in normal Python, and still has all relative downsides of dynamic typing. To be frank, I don't get the point of using PyPy instead of Go.
Previously most Python libraries already use C to optimize critical components. If you can use just one language to get similar performance, why do you bother using two then?
Github should really just enable the developers to specify the language of their repo by themselves. Bitbucket get this right in the first place. Auto detection for language sounds cool, while doesn't work for most of the web project.