It's interesting to see how PostgresSQL didn't get nowhere near the popularity of MySQL. I think a few major things contributed to this:

1) MySQL focused on performance first while PostgresSQL positioned themselves as "feature-complete."

Of course, that was years ago and in the meantime they became comparable in both aspects. MySQL+InnoDB is ACID compliant and Postgres matches the InnoDB performance.

Sure, Postgres might still have the upper hand in the variety of features but MySQL has MyISAM. If you don't need transactions you can get a significant performance boost over Postgres. Speed IS a feature and one could argue it's more important to many than all the nice extra things Postgres has.

2) MySQL was compatible with mSQL so they got a pretty good user base right from the start. mSQL later became obsolete.

3) Inertia. Even if Postgres would be net superior to MySQL (think feature complete but faster than MyISAM) it would still take a lot of time to catch up, if ever. There are tons of tools already developed for MySQL and a huge knowledge base around it. Just like C++, Windows, or any other big technology, there's a lot of inertia to it.