Errm why?
Perhaps they have loads of shipped but unsold stock sitting in warehouses gathering dust. That's what happens when you fudge NPD numbers to create hype. You stuff the chain with product they don't want and hope the resulting hype will eventually sell them all..
However this time it didn't work, consumers very quickly found out Kinect was useless and expensive shit, with serious lag and detection problems and an accute software problem with little more than a handful of jumping and waving games (something that's still not been rectified a year later).
Microsoft really did create a hardware Bob...