Because huge amounts of code are already written in PHP/MySQL and it's already installed on many webservers.
(oh and one small word: Wordpress)
Which, continuing wildly off-topic, probably means that Postgres is probably a sensible option for a bespoke system coded from scratch but most of us in a production environment need to use frameworks.
A framework, to be database independent, making a switch to Postgres feasible, would need all database calls on an abstraction layer. Unfortunately given the large user bases of such frameworks, moving such juggernauts means we end up with MySQL (it does the job) rather than Postgres (it does the job better).
I am also a fan of keeping textual content separate from graphic content and, punching above my pay grade, seems to imply MySQL plus a good webserver makes a lighter solution than an integrated database using Postgres.