WordPress is not a very modern PHP CMS and does not use Symfony or other components. It is very popular and earns it's position in the listing of Symfony CMSes.
WordPress has grown from a blogging platform to a system that is used to power all types of web sites. The technical quality of WordPress might not be as good as modern CMSes, but the user experience remains very good.
WordPress also relies heavily on external plugins for core functionalities like multilingual. When using a lot of 3rd plugins can lead to a very brittle and unsecure WordPress installation.