I would guess one of the reasons to be the fact that there are a myriad of (money-free) hosting providers out there offering plain drag&drop file uploading with PHP support (often paired with a mysql database access). This makes PHP software extremely easy to install.
Not that it would be even possible to run non-PHP server side scripts with majority of said providers...
Not that it would be even possible to run non-PHP server side scripts with majority of said providers...