Possibly because they didn't intend for it to be used for motion sensing which isn't surprising considering the motion sensor and screen are attached to the same device.
Nintendo could have added SIXAXIS type functionality into the DSi. They still could on a successor system...
No, they can't add Wii style motion controls on the DS, unless it was with a peripheral of some kind. I don't see why a tiny wiimote kinda thing on the DS would be impossible.