Another thing that has only been mentioned obliquely is support. When an
organization selects an enterprise wide mission critical database system
they get a lot of perceived security from purchasing a world-wide 24x7
support contract from a company with the resources that Oracle can
provide. Yes, you have to write a big check but you are insuring against
a big risk. As others have stated, many of the factors that have a major
influence on the choice of a database are not technical.
Bill