Canon currently outsells Nikon, and has, for a number of years. Part of it, I think a large part of it, goes back to around 1990 when auto-focus technology burst on the scene and, simply put, Canon had the better, faster, more powerful auto-focus technology. A lot of professional sports photographers at that time, dumped their, now-slower Nikon kit for faster focusing Canon kit. Nikon caught up at some point but in this race, but not before, droves of pro's had made the big switch to Canon. With no compelling reason to switch back to Nikon the sidelines have been packed with white lenses and that's pretty powerful advertising.
Another thing I've noticed is Nikon color versus Canon color. Nikon cameras tend to run on the cool side, meaning shots from Nikons tend to be a little too blue. Canon's, on the other hand, seem to run a little on the warm side with a little too much red/orange. Given a choice, and all other things being equal, people prefer to warmer colors. My Nikon's shoot so cool part of my normal post processing routine is to check, and reduce if necessary (which is needed far more often than not) the blue mid-tones by about 5 points (using a Levels Adjustment Layer). Check it yourself and see if I'm not right about this... The cool tones are subtle, but they're there.
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