I wonder the same thing. I have not programed a DB but..
I would think the worst thing to program in a gc'd language would be the cache. So, if you write that layer of the database separately in a non-gc'd you would avoid most of the headache.
EDIT: I read the parent comment as why program a db in a gc language. I think the new wave 1st gen db's are doing alot of novel things. Minus the cache I imagine the velocity improvements form the 'simpler' language makes sense. Key value stores are become well trod ground however.
I could see the opposite working out very well. Having a GC type you can box other types in.
https://github.com/Manishearth/rust-gc