Nobody in the core development team has any significant experience with databases.
By not providing a database abstraction layer for the various storage pools (including mempool) they can have a better grip on the whole project.
Check out the historical perspective from 3 years ago:
blah blah blah blah
Gentle reminder to the other bitcoin developers: it is generally best not to feed trolls. Use the ignore button.
Edit: Actually I just realized that you've asked about VSAM, which would mean that you've compiled it on the (big-endian) IBM mainframe. Does it work at all?