It could be that.

I wonder if it's also just easier for people already familiar with git.

I've tried running a Wordpress site for example, and I thought it was a steep learning curve. Even now, after setting up the site, I think it would be easier and faster for me to be pushing markdown files to Github than to use my site.

Of course, it's a lot less powerful, but if you just want to get decently formatted content onto the internet fast, it seems like a good option.