We wanted to pick a connector that was easy for owners to obtain and solder. We considered a DE15HD 15 pin high density connector commonly used as a VGA connector, but the high density made it more difficult to solder. We thought about a regular DA-15 commonly used as a game port connector, but they are harder to get and take more back panel space. We could have used the extra pins of a 15 pin connector for some additional features. Instead we compromised with the DE9.
For more on the configuration of D-subminiature connectors, see this page: