It's more expensive to put a game on a retail shelf for many reasons, not just the retailers cut. Steam also takes a cut. For retail you have to manufacture,

It's more expensive to put a game on a retail shelf for many reasons, not just the retailers cut. Steam also takes a cut. For retail you have to manufacture, you have to distribute (world wide), you have to maintain an inventory, you have to get packaging ratings and comply with retail rules. It only makes sense to do for the titles you have the most confidence in. Which tends to be the premium ones.