Not to assume, but that sounds like a comment from someone who only uses GitHub for interacting with git. I agree that GitHub helped spread the popularity of git, but I hesitate to say with absolution that it's the only thing that makes git popular.
Anecdotally...
When I first began my git adventures, nearly 10 years ago, I was extremely apprehensive. I knew SVN. I liked SVN. I had no reason to go elsewhere. And then I tried git (albeit using a GUI at the time), and it didn't click. So I went back to SVN. Later, after being forced to use git for a project at work, I started to understand what makes it special. I finally felt I had a version control system/tool that I could trust without being so hands-on like I was with SVN. Put simply -- git _just_ works. The concepts take time to really grasp (rebase versus merge, reflog, etc), but once grasped, it becomes very easy to see why it is so popular.
I now use git locally as much as I use it with a hosted repository (GitHub, GitLab, BitBucket, etc). I use both command line and GUIs, and I thoroughly enjoy being able to trust git so completely.
Git was popular before GitHub was a thing. Git was taking over the open source world back when everybody was on SVN, and people were just hosting their Git repos themselves. GitHub made Git ubiquitous for all types of developers and accelerated that adoption further, giving us the near-monopoly we see today.
A better statement is that Github rather killed the momentum of Mercurial, which was also picking up steam in that era (Python, OpenJDK, and Mozilla all chose Mercurial over git).
