The reason is that GitHub, being GitHub, has a lot of search indices and existing analytics on the code that is hosted on GitHub.
It was easier, at least - but probably gives some nice guarantees about the statistics of "public" code, with the norms and conventions you're "used to", because you're used to the internet's coding norms.
Company code can be pretty bad, even at Microsoft, often riddled with hyper-verbose variable names and strange design patterns.
