Git became popular because it was one of the first two open source distributed version control systems. Compared to the least-bad open source (non distributed) version control system before, SVN, the native branches and the ability to have a local copy of the whole tree were self evidently a revolution.
(The other one was Mercurial by the way, released at almost exactly the same time as git. Partly git won that race because of the cachet of being written by Torvalds and being used for the kernel, but I suspect mainly it was due to the existence of GitHub.)
Aside from the above, it's also just clearly not true that git flow was particularly common. It's no good claiming anyone that disagrees is in a bubble. We all have access to GitHub! Look for yourself at some random repos (and make sure you sample a few different languages). It will verify my experience of looking at dozens, probably hundreds, of repos over many years: the number of people using git-flow is, to a first order approximation, roughly zero.
