[Fnoord]

The reasoning isn't "it is owned by Microsoft"; that is just a trigger. The reasoning is thoroughly mentioned:

"[..] But independent of that, GitHub has always had a vendor lock-in with the user's issues and pull requests hidden behind a rate limited API instead of a proper export feature. And even if you managed to export it through that API, you can not host your own GitHub instance and modify it as you like because there is not even a partially open source version of it."

[neilsimp1]

I'd love to see Microsoft turn the tables and open source (at least partially) GitHub. I don't think it'll happen but it'd certainly get some interesting responses.