I read this a long time ago, and I believe it still is true: Nikon hurt itself terribly by reducing the support to professionals, or just not keeping up with what Canon did to support them. As a result, there are a lot of Canon shooters at high profile events with the distinctive Canon off-white lenses. This visability has helped Canon capture a lot of the amateur market share, and it is the one that sticks the most; i.e., amateurs aren't as likely to switch out gear brands as professionals are.
Couple this with the mirrorless market surge, which in my casual eye is set to be dominated by Sony, and it is no surprise that Nikon dropped to where it is. The question I have is when Sony becomes the overall leader is camera distribution, outside the cellular phone market.
WM