[matthewmacleod]

Postgres is widely regarded to be a good example of an open-source project – technically excellent, with a good codebase and community. MySQL has a history of being less technically sound – although it's certainly improved, I imagine it's left a bit of a legacy. In addition, the purchase of MySQL by Oracle probably makes many developers more skeptical of using it.

IME there isn't really a compelling case to use MySQL over Postgres, unless there's a specific environmental constraint.