I think it had more to do with GitHub than anything else. In GitHub's early days, they put a lot of effort into marketing Git (e.g. the now defunct "Git Is Better Than X" website), because they needed people to use it in order to use GitHub.

I reckon it was a combination of Github and Rails being early adopters at the peak growth of their hype cycle. In the early days of Github it seemed like most of the projects there were Ruby based (Rails users overflowing).

Developer word-of-mouth in general, combined with the massive network effects of a version control system. Once you use a VCS because software you work on uses it, if you want to start your own project, you tend to use what you know.

Git was designed for the Linux kernel source which everyone using android or another Linux distro is interested in. That helps a lot because if it can handle that project surely it can handle any other codebase.

Not any other codebase. Codebases with huge and often-changing binaries (e.g. images or 3d models), for example, were troublesome for git, at least historically. The Linux kernel isn't big in all ways.