For several reasons. Back when the Xbox division was run by complete ass clowns, they didn't care about adding in backwards compatibility because they really didn't view the Xbox One as a video game console. To quote Don Mattrick "If you're backwards compatible, you're really backwards." I think that backwards compatibility would have been included if Phil Spencer were in charge of the division when it was in the thinking stages.
There is also the matter of cost. Because games are getting more and more complicated and advanced, the Xbox One would have most likely required Xbox 360 parts in order to effectively play Xbox 360 games. Pure software emulation of Xbox 360 games is still years away from being decent. Microsoft brass most likely didn't want to absorb the extra costs of backwards compatibility and with the Kinect sensor already adding $100 to the price tag, I doubt they could have gotten away with passing the extra money of the Xbox 360 parts onto consumers.
And there's also the matter of technology, Microsoft has come out stating that because the Xbox One's architecture is completely different than that of the Xbox 360, there are a lot of challenges associated with bringing backwards compatibility to the Xbox One. With the Xbox One, Microsoft transitioned away from the PowerPC architecture that powered the Xbox 360, back to the x86 architecture that was used in the original Xbox. The CPU is different, the memory is different, the operating system is different.
That said, Microsoft is currently looking into bringing backwards compatibility of Xbox 360 games onto the Xbox One the same way Sony is rumored to be bringing PlayStation and PlayStation 2 games onto the PlayStation 4. I would be surprised if they didn't include Xbox games as well.