I'm speaking completely out of my ass here, but my suspicion for why PHP took over the web is largely because it was "web-first". It was free, open source, and

I'm speaking completely out of my ass here, but my suspicion for why PHP took over the web is largely because it was "web-first". It was free, open source, and you wrote directly on HTML pages, instead of Perl spitting out HTML via CGI. I think ColdFusion was briefly popular for the same reason (though I think that one fell out of favor largely because it cost a ton of money).