My impression is that there are more Canon users than Nikon users. This year, the group I'm working with on Friday nights is 5/3 Canon/Nikon.
I'm willing to bet that it has something to do with the amount of television advertising that each company has hired over the past 30 years or so. I seem to remember a whole lot more Canon ads than Nikon ads. In fact, the earliest television ads between the two that I remember are all Canon. I believe that Nikon has put more of the ad resources into the print media. (An example is while I remember ads for the Canon AE-1 Program, I can't recall a single ad for the Nikon FE on television.)
I could be mistaken, but it's what I remember.
WM