The choice of PostgreSQL comes down to a combination of personal preference and the existence of a wider range of missing features than any other database Django supports.

Like Oracle DB? ;) Though, to be fair, I haven't heard of many Oracle backed Django instances.

I think that this is a good way to go about broadening a particular DB's support: implement in contrib, and make the general ORM capable of utilizing such contributions when available, while retaining regular DB support.