1) Canon has a shorter flange to sensor distance. Mechanically u need the canon rear of the lens to sit much closer to the sensor than the usual nikon system to achieve infinity focus.
Of course the 'ugly' alternative is someone actually comes up with an adapter with an optical element to correct for infinity ..
2) plus. all electronic contact of the canon and nikon lens/body will not be working, meaning u cannot control the canon lens aperture and u will be always shooting at the smallest aperture.
( although there has been a nikon-G to EOS adapter that controls the aperture somewhat, the point numner (1) will make the manufacturing of such an adapter pretty not feasible )