Because MySQL's history is one of specializing in content management. In fact if you look at things this way, many of the MySQL gotchas which make DB people cringe are actually content management features. Yes, this includes data truncation.
When you move off the web site/content management side, PostgreSQL has long been the open source DB for complex business tools.
Because up until recently PostgreSQL replication didn't really exist, and you had to choose between 10 different trigger-be-using unscalable bag-on-the-side technologies to fill that void.
Still I know of at least one quite large website (one of the largest in Sweden) that use Slony despite the performance hit from trigger based replication.
Betamax/VHS (look it up). MySQL had earlier presence, perhaps because it was easier to install? Hence people expected to find it as it was what they knew.
MySQL had earlier presence because it was the only available solution. PostgreSQL came much later. There was something called postgress95 which crashed on connection.
Why is that ?