Tool usage patterns often exhibit social clustering. I.e., friends and colleagues tend to use similar tools, through a mixture of exposure, recommendation, similar work habits, shared constraints, etc. When the tool in question is about interacting with others (whether socially or professionally), it's naturally even more pronounced.
I found it pretty popular in parts of the python community. And I guess it's used for a lot of private repositories.
exactly, private repos. When you are talking about some OSS project it is often on github, but the authors might still have their private stuff on bitbucket. And there would be no reason to mention it.