The Word of Notch has, well, the word of Notch on why Minecraft is not sold on Valve's Steam service. While saying it's the "best digital distribution platform I’ve ever seen," he also explains:

Being on Steam limits a lot of what we’re allowed to do with the game, and how we’re allowed to talk to our users. We (probably?) wouldn’t be able to, say, sell capes or have a map market place on minecraft.net that works with steam customers in a way that keeps Valve happy. It would effectively split the Minecraft community into two parts, where only some of the players can access all of the weird content we want to add to the game.

We are talking to Valve about this, but I definitely understand their reasons for wanting to control their platform. There’s a certain inherent incompatibility between what we want to do and what they want to do.