The incompatibility is due to the fact that the Xbox One runs on x86 processor architecture, whereas the Xbox 360 ran on PowerPC. This fundamental difference in hardware architecture prevents the Xbox One from natively running Xbox 360 games, regardless of how powerful the thing may be.
"We care very much about the investment people have made in Xbox 360 and will continue to support it with a pipeline of new games and new apps well into the future," a Microsoft representative told Engadget. Part of that investment will transfer, however: Your Xbox Live Gamerscore.
Earlier this year, Sony also announced that its PlayStation 4 will make the jump to a processor built on the x86 platform.