I remember that PHP became popular because of mod_php. If you wanted to script a web application on Linux or UNIX your choices were either Perl with CGI or a much faster PHP with mod_php.
It was incredibly easy to setup both in production and for development - hence the LAMP stack and the thousands of cheap shared hosts that had PHP enabled, which lead to a lot of pre-install applications being written in PHP.
Other web applications only recently became easy to install, and they didn't beat 'ftp into the site and drag files over' until cloud platforms and virtualization.
They still haven't beaten PHP in ease-of-deployment. I've been writing for the web in Python for 2 years. Deploying your own server is a full-time job.
The only thing that's getting better is that things like Heroku now exist. Heroku makes it insanely easy to deploy Python, compared to deploying on your own server. It's still harder to do than FTP + copy files, which most people know how to do.
It was incredibly easy to setup both in production and for development - hence the LAMP stack and the thousands of cheap shared hosts that had PHP enabled, which lead to a lot of pre-install applications being written in PHP.
Other web applications only recently became easy to install, and they didn't beat 'ftp into the site and drag files over' until cloud platforms and virtualization.