Designing your own hardware (and the entire OS on top of it) is expensive. Using off-the-shelf components and an Android base will help that immensely. If Ouya can manufacture its console for slightly less than its $100 price (assuming the kickstarter pledge level to be the price goal), it can build its profits off the game store.
If they can make it reasonably easy to Ouya-fy an existing Android game, there will be reasonable incentive to add games to the store. In my mind, this is the riskiest part. Tablet and phone games are designed around touch. Many of them won't convert to a controller well, which will limit the number of games ported and make them more dependent on made-for-Ouya titles. They'll need to be a good-sized market for that.