GameRevolution: "The Xbox One has built momentum quickly in a way that has defied all odds. Some are wondering how such a radical shift could happen so suddenly. Although the answer is simple, it includes multiple factors, each of which we'll discuss below."
The biggest reason, which they didn't mention, are the fire sales of the old model. There have been a few every month since July. I'm surprised retailers still have stock. Microsoft must have shipped an insane amount.