One reason which I heard Canon was more popular than Nikon deals
with history. Canon provided a highspeed telephoto for sports
before Nikon did, so once people bought into a system they stayed
with it.
Another reason might be Canon provides more. More lenses with USM,
more market share, better point and shoots. Well atleast this is
what adevertising leads me to believe. Canon is in everything that
you would need, cameras, lenses, printers, and you can even invest
in Canon. Nikon is owned by Mitsubishi company.