I had to believe Canon is more successful with their marketing. And they focus on both serious photographers and people who's just getting into the hobby. Look at their point and shoot line up and their entry level DSLR. They're virtually everywhere.
At certain point, Canon camera were better at delvering high ISO images, and they always have a larger selection of lenses. And they've always put a lot focus at marketing their cutting edge technology.
Most of the people I know who buy into Canon are chasing the latest technology hype. The only problem is that the camera that you buy will only have the lastest and greatest for a couple of years ir not a couple of months.
I like my Nikon cameras because of the ergonomics, the way the controls/buttons are layout. I am never a big fan of buttons and on screen controls. It's almost like the race between windows and Mac situation. (Canon more like Windows, latest and the greatest with all the bells and whistles while Nikon is like Mac, it just works)
when you're buying into a brand, you are buying into a whole system. The lenses and accessories often cost more than the body. It is a fact that technology will not stop advancing, (better AF, better high ISO performance) I try not to invest in the most expensive camera body and put my money into my lenses. The Canon bodies might actually have better features, but in order for me to switch, I'll have to loose thousands of dollars of investment and I might as well spend that money on a D4 if I'm going after those technical specs.
It does however makes sense for people who is just starting the hobby and that's exactly market sector Canon is going after.