Minecraft has been financially successful, in spite of avoiding the most common digital distribution channels. Valve's Steam service is an especially notable absence, and creator Markus "Notch" Persson explained why we haven't seen the game hitting Steam yet.
"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," Persson wrote on his blog.
That doesn't mean Minecraft will never come out for Steam, however. "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."
If this rationale sounds incredibly familiar, that's because it's the same claim EA has been making recently. Steam has adopted a policy that doesn't allow for new in-game storefronts, though old ones are grandfathered in. EA has been bucking against this policy with several high-profile games that feature in-game storefronts for DLC.
Dragon Age 2 and Crysis 2 were pulled, and EA has already opted not to release Battlefield 3 for the same reason. In each of the cases, EA has referred to "restrictive terms" that "limit how developers interact with customers."