On the subject of why LevelDB is used, core developer Greg Maxwell stated the following to the ref:

I think people are falling into a trap of thinking "It's a <database>, I know a <black box> for that!"; but the application and needs are very specialized here. . . It just so happens that on the back of the very bitcoin specific cryptographic consensus algorithim there was a slot where a pre-existing high performance key-value store fit; and so we're using one and saving ourselves some effort...