elephants is about keeping track of what you're doing to have a continous memory of your day. It's like the narcissism of twitter, but for your eyes only.
No reason. It's hip and happening now. You could use anything as the database. You're just saving timestamps and strings at the moment. It could use a csv for all I care. I'd like to refactor it so that the database layer is appropriately abstracted away so you could use anything, but I'm short on time lately.
Why not... MongoDB?