Until recently, the major database vendors did not offer a full suite of database security options. Now they do, with activity monitoring, assessment, and transparent encryption to go along with access controls and authorization schemes. Rummage around the open-source Web sites and you will discover deployment guides and basic security tips -- but these center around access controls and secure communication (i.e., SSL). Security research and tools designed to help secure these platforms is generally absent. Third-party vendors have not ported their monitoring, masking, auditing, and assessment tools because customer demand has not been high enough to justify the costs.
To get a better idea of why this is important, let's look at vulnerability assessment. There are no formalized assessment products, and beyond a smattering of policies for MySQL, no research teams perform policy development for open-source databases.