I stand corrected then. They dominate the market, but it isn't because their products are better than everyone else... its because they're the most used by everyone.
(Personally speaking, I believe their products are better than everyone else, because they have been premiere makers of imaging equipment for God knows how long)
Back in the day with 35mm, I think the market was much more competitive. I shot with an Olympus OM-2 for the longest time... so you could make more of an argument for the Olympuses, the Pentax, the Minoltas, etc....
However, since the transition into the digital age, I believe that Canon and Nikon have emerged as a leaders. Call it better advertisement, or call it what you want, the fact remains that they are the leaders of the digital imaging industry.