1. It was simple to use in an era where CMS meant 'large, complicated system that was tailored towards technies rather than the general population'. It didn't have a ton of features, but it was easy to use for the average blogger wanting to write posts without worrying about all the setup and coding and what not.
2. You could pretty much install it anywhere without minimum technical skill, since PHP and MySQL was (and is) supported by about 99.99% of shared hosts. Its newer competitors tend to be written in languages that are usually only available on a VPS or dedicated server.
3. It has a very large ecosystem for themes, plugins and other such things, so someone with no technical skill could find add ons for anything they wanted without needing to call in a developer or know PHP.
It's not the best written software and its not the best optimised for anything in particular, but it is one of the easiest to install and use for someone without a technical background, which is why it's so widespread.