Why isn't Minecraft on Steam? The developer of the game, Notch, has probably gotten asked this a lot, and after answering the question for some people at PAX, he decided to be a bit more open about the entire situation on his blog today.
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.
So, no hidden conspiracy, just not a good fit at the moment. And he even mentions that they're still talking to Valve about bringing the game over to Steam. After all, Minecraft is coming to the 360 soon, so we know he must be willing to make at least some compromises.