Because PHP and the whole LAMP stack is like air. Everyone knows it because all you have to do to use it is just breathe. Anyone can just pick it up and immediately make an awesome website. It's a simple easy-to-learn language with dozens of easy all-in-one installers. No command line haxoring needed. Unlike ASP.NET, the tools are free. Unlike Rails or Django, hosting is cheap and easy to find.

With all the hate on PHP, I feel like it's the childhood friend from elementary and middle school that you out-grew because you became "too cool" for them in high school.