1. Branding. First and foremost. MySQL had great documentation, an identifiable logo, and a wonderful name that every non-technical person could remember and identify. Postgresql is just, well, it's a horrible nightmare of a name. People would like to think this doesn't matter but: of course it does.
2. A really slick installer for Windows. You could say this is branding and to some extent it is, but MySQL and its admin tools have a really friendly installer that lets intermediate developers without a lot of DB experience get it up and running with their development environments quickly. I admit it's been a while since I used Postgres's but the fit and polish wasn't there.