Comment by threeseed

1. It's far more popular so easier to find solutions to problems.

2. Used by very popular websites e.g. Twitter, Facebook, Yahoo so you get projects like http://webscalesql.org to utilise.

3. More options for client side tooling although debatable whether these are better than generic options.

4. MySQL Cluster is a more cohesive solution for clustering.

5. The most likely reason: you have bought something that needs it.

Obviously PostgreSQL has plenty of reasons for choosing it over MySQL.