Because these apps exist. WordPress, for example, is unparalleled in other language domains. What is the WordPress of Ruby? And in python? Is it Django? Where's the plug-in directory for Django, equivalent to [1]?

I work with WP, I'm the first to recognize it's an old codebase with all defects this entails, but popularity has value. Value that PHP critics dismiss too lightly.

[1] http://wordpress.org/plugins/