The PHP API is incredibly direct and there's little development overhead.
Postgres on the other hand always felt kludgy for small projects. Just getting it up and running and creating databases took longer than with mysql. And phpMyAdmin is a tool most people are familiar with, for which I can't even think of the postgres analogue.
Pardon my butchering of the phrase but postgres is a hammer and mysql is a scalpel, and most projects call for a scalpel.