Perhaps the main reason WordPress is so popular is that it is easy for web designers/developers to produce a website using WordPress.
Why WordPress is the best
WordPress can be installed for free with 3 clicks on cPanel website hosting.
WordPress has 10s of thousands of free and commercial themes to choose from.
Web page content can be edited quite simply by the website owner while maintaining the design of the website, once the website has been handed to them by the designer/developer.
Websites can be migrated between different web hosts quite simply too.