This is the biggest factor. Canon Professional Services offer extremely good support for their cameras. Nikon Professional Services are good too, but not quite

This is the biggest factor. Canon Professional Services offer extremely good support for their cameras. Nikon Professional Services are good too, but not quite as good as Canon's. And Sony... is lagging very far behind. Canon also is known for top notch lenses, which is the real core component of a camera system. You may spend $2k-3k for a camera, but as a professional you'd spend much more money on multiple lenses you need that are all $2k+ each, and you'd be using the same lenses each time you replace the camera body. Canon cornered the professional market this way.