[wsprague]

The trite answer:

Because idiot MBA managers have heard of MySQL but not Postgres. (Sorry -- I couldn't resist.)

The longer answer:

When the internet took off, MySQL was a lot better than Postgres in terms of stability and speed, though not in terms of features. It grabbed marketshare and "mindshare" then, and it has retained them, even though now Postgres is its superior in terms of features, concurrency, and stability. Because of the marketshare snowball effect, it also remains far easier to hire a MySQL tech and to explain MySQL to a non-technical manager.

MySQL also continues to lead in the realm of simple andd effective replication, which Postgres does not have yet, though they are working on it (furiously).