For the same reason Sony almost bundled their camera with the PS4. By making it mandatory, you can (theoretically) get devs to utilise it for something more than throwaway party/casual games, because you know every user has the hardware. Sony made the call to keep the production cost lower by leaving it out and thus have a lower retail price, MS is taking the gamble that bundling it will have a payoff via some exclusive killer app.
Given what a gimmick I consider the whole motion control BS to be, in the same league of uselessness as 3D tv/movies, my personal feeling is that Sony made the correct decision in this instance. But then again apparently the average slob loved the Kinect, to hear MS tell it. So how it plays out in the long term I guess we'll have to wait and see.