Xbox One Games At E3 Were Running On Windows 7 With Nvidia GTX Cards
1) Microsoft isn't confident enough in Windows 8 to use that as the development OS.
2) They are using a GPU from a different vendor, which means that games cannot be getting AMD specific optimizations out of the gate.
3) They are running games on a CPU and GPU which are so much ridiculously more powerful than what is going to be in the Xbox One, so you can expect games to look significantly worse than those demo'd at E3.
4) Microsoft is writing the book on PR disasters with regards to console launches.