Github has traditionally been a Ruby shop and once you are a Ruby shop, you can use Ruby to do anything that you could use Python for so there is no need to touch Python other than may be data science. That means they would have built a lot of expertise in Ruby and comparatively very little with Python. So it's understandable that they are able to add support for Ruby before python. It must have been easier to do for them and also been much easier to "dog-food".

That said, I'm sure Python, Go, Rust and other languages will be supported very soon.