yo_john117 on 25 May 2013
Because:
It's starting over from the 360. Fresh, appealing, perfection.
Xbox One hints that the Xbox is going to win (hence one or won). This way instead of saying Xbox 720 won, you can simply say Xbox One/won. Simpler, easier, and better, much like the system itself is.
It's a sign to HTC that MS is going to buy them so they can have quality, exclusive Windows phones that are named HTC One (and essentially ending the android version of the HTC One).
The number one is the first positive number. They want to show that before anything else, there must be one.
They want to show the world that they are the number one in everything ever.
