Because those of us who know and have used several languages tend to develop a preference for expressiveness, terseness, safety, and dare I say, elegance.

PHP (and JavaScript) gained and maintain their relevance, despite being inferior languages, because of their historical placement. I'm sure COBOL has improved over the years, and we know it is still used and "needed"... because of its historical placement. But you probably wouldn't use it, either, unless you had to (or you didn't know a better alternative).