It's really, really hard to build an open source (free) product that makes any money. GitLab works because it's a very complex product that a few really large companies are willing to spend hundreds of thousands of dollars on it. I can't imagine that being true of a CMS.
That being said, different genres of software have "known" price points. For source control, it's expensive. For analytics, it's expensive. For blogs, it does seem like the open source model is the only one that's really taken off... Wordpress and Ghost both use the model, and (for better or worse) anchored the price point of blogging software somewhere between free and $10/mo. Makes it hard for any incumbents, which is probably why WordPress still is so popular.
That being said, different genres of software have "known" price points. For source control, it's expensive. For analytics, it's expensive. For blogs, it does seem like the open source model is the only one that's really taken off... Wordpress and Ghost both use the model, and (for better or worse) anchored the price point of blogging software somewhere between free and $10/mo. Makes it hard for any incumbents, which is probably why WordPress still is so popular.