Because the PS3 is more powerful when you design and code for it. The Xbox wouldn't handle the graphics
Uncharted is graphically designed for PS3. It wouldnt look as good on Xbox. Plus, yes, Sony does own ND as well as other companies, like MediaMolicule, Suckerpunch, and a lot more.
I personally could never see Uncharted on Xbox. Just looking at the gamecase tells me: Playstation.