Nikon has been less reluctant to put pro features in their "semi-pro" cameras. Thus we have many pros using cameras like the D700, D300, and D7000. Canon has held more features specifically to the 1D and 1Ds cameras (notably their best autofocus systems have been withheld from 5D cameras until now).
I think Reichmann's point is right: Canon is #1, so there is a tendency to iterate slowly and stick with what has been working so well. The Powershot G series has been an overwhelming success, so why change much? Get a bigger sensor in there, lens to match, and keep the rest, good (build, controls, JPEG engine, display, etc) and bad (viewfinder, autofocus speed, etc). As noted above, Nikon is a bit hungrier than Canon (also evident in the D800 pricing which is $500 less than the 5D III pricing), and Sony/Panasonic/Olympus are hungrier still.