Using BitKeeper as the SCM for the Linux kernel always seemed like a bad idea and when issues between the company and the community peaked git was created.
https://git-scm.com/book/en/v2/Getting-Started-A-Short-Histo...
https://git-scm.com/book/en/v2/Getting-Started-A-Short-Histo...
Bitkeeper itself is open-source these days available via the Apache 2.0 License, but it is too little, too late: