I'd guess easy hostability would be the main one, not anything technical. More hosting companies support PHP than any other language, and using it means people can set up their own file sharing system without needing to know much about server management.
That's arguably the reason most popular CMS systems run PHP too; because they'd rather have something that's easier to setup and can be run nearly anywhere than something that requires knowing how to use a terminal or something.
I don't think shared hosting is dead. It's less popular among startups and tech savvy companies than it used too be, but there are still tons of companies and individuals who use it. Companies like GoDaddy and EIG still make a ton of money (even if many would rather they didn't), and many agencies specialising in WordPress/Magento/Drupal/whatever still get a lot of business.
That's arguably the reason most popular CMS systems run PHP too; because they'd rather have something that's easier to setup and can be run nearly anywhere than something that requires knowing how to use a terminal or something.