Why? Besides

the obvious advantage of full utilization of existing wide and super wide angle lenses, 1Ds and 1Ds MKII are targeting studio photographers to replace MF. Of course a digital back can be used there, but it's much more expensive. The 1Ds is also a superb nature photography tool, combined with Canon's army of lenses, you have a super camera to shoot landscape, birds, macros and other stuff. Canon's strategy is to conserve its line of lenses and design bodies to utilize the lenses to maximum (the 10-22 and 18-55 lenses not withstanding) because of lenses the existing users have already had. Nikon would like to do the same but has not figured out how to make full frame sensor yet. So Nikon's strategy is to design new lenses to fit for smaller sensors. Other players do not have full line of lenses so they can afford newly designed bodies on smaller sensors combining with new lenses. Of course the drawback in Canon's approach is heavy and bulky systems compared with Olympus's 4/3. But hey, Olympus has not made a high res sensor yet and could never be able to because of its sensors' small physical size. 300D, 20D, 1D MKII and 1Ds MKII cover almost the full user spectrum on DSLR market. It looks like monoply.