Well, it comes down to a few things really:
1. These CMS systems made it easy for non technical users to add or update content. That's important given that for these people, even stuff like Git is seen as confusing and archaic, whereas 'go here, use an editor that looks a bit like Microsoft Word, publish' is easier to get the hang of.
2. Many popular systems like WordPress and Drupal ended up creating a large ecosystem around plugins, themes, etc. So for a non technical user, it was easier to just download or buy a plugin/theme that does what they want rather than have someone code it from scratch or bridge it with another system. It also led to companies where their only skill was to either build CMS plugins/themes or to install existing ones, and those companies then had a good incentive to suggest the same system for their future customers.
3. Shared hosting was/is still easier to understand for many people than a VPS or AWS might be. Even now, what do you think people are more likely to understand? Press a button in a tool like CPanel to auto install a CMS or manually set up a static site generator themselves? Stuff like Docker or containers or Git or SSH or whatever else all seem like black magic for many smaller development agencies, let alone end users who have no idea what a CMS actually is.
4. If you wanted dynamic features (like a comments system, poll, user login/account setup, etc) then you needed a CMS or programming knowledge. Hell, even today you need one of those if you don't want to be stuck relying on a third party service for your website. Do you trust Disqus? I sure don't.
5. Finally, I guess popularity just begets popularity. People like jumping on bandwagons, and once WordPress started growing in popularity, that meant a whole bunch more people wanted to use it as well.