Minecraft isn't on Steam mostly thanks to Valve's restrictive terms of service that prevents publishers from releasing additional content through other platforms other than Steam, said Minecraft creator Markus "Notch" Persson.
"Steam is the best digital distribution platform I've ever seen," said Notch in his blog post. "I've spent incredible amounts of money on it, and I own a crazy amount of games on it. It runs great, offers great services like that shift+tab stuff, and it remembers my credit card details so there's no barrier for me when I want to buy a game. The only downside I can think of is that offline mode is a bit flimsy, and that the game list is sometimes full off DLC releases for stuff I don't even own, and those are some tiny complaints!"
"But," he continued, "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.
"So there's no big argument, we just don't want to limit what we can do with Minecraft. Also, Steam is awesome. Much more awesome than certain other digital distribution platforms that we would NOT want to release Minecraft on."
Source: The Word of Notch