Of websites using a discernable server-side programming language, over 80% of them use PHP -- https://w3techs.com/technologies/overview/programming_langua... -- The four biggest content-management systems on the web are all PHP -- https://w3techs.com/technologies/overview/content_management...

Accessibility of language, number of developers, and hosting resources. Quality is a different story.

Ease of setup, I think. Almost every web server in existence can run PHP.

Ease of deployment. LAMP stack was PAAS before PAAS was a thing.

a lot of these systems were built back when PHP was hipper. Despite its drawbacks PHP is the energizer bunny of web tools...