It was super simple for me. I was 18 years old, needed to get my first SLR so i could stop borrowing my parents and Canon was less expensive than Nikon, and the MAIN reason back then was the weight. Back when they were SLR's, Canon's were a lot lighter weight than the Nikon's were. I used to work at a Ritz Camera just so I could get the discount on Film and Processing back when it was 35mm, and while trying to sell the two side by side, most customers would say "The Canon just feels cheap because of how light it is." The Nikon's were all metal and really heavy
Cindy and I were huge in MTN. Biking, Hiking and Climbing and I wanted the least amount of weight hanging from my neck as possible.
Fast Forward 20 years and we now have amassed a huge collection of Canon specific gear. We don't get paid by any company so don't think I'm making this statement due to image quality, but if I was just starting out, I'd choose Canon simply due to lens cost. I agree with everyone else who says the image quality and pixel peeping between brands is silly these days. It is what it is and any one of the professionals named in this thread could produce cover material with either brands.
One thing I have noticed since we've been running guided photographer tours is the Nikon's are super hard to navigate through when it comes to their menu's. With a Canon body, I can usually change most of the setting's I'd change throughout the day without having to go deep into any of the menus. With the Nikon's, I usually have to search for it and cant do it from the back of the camera with just a quick button like the Canon's have.
Here is probably the main reason
Canon 24-70 f/2.8 - $1599
Nikon 24-70 f/2.8 - $1886.95
Canon 24-70 f/2.8 II - Their latest Version - $2299 - Crazy that they come out with a new version that is this much more!!!
Canon 70-200 f/2.8 - $1449
Nikon 70-200 f/2.8 - $2396.95
Canon 70-200 f/2.8 II - Their Latest Version - $2299 Again, crazy for their lenses to come up in price this much, but again reminds why every photographer will say "Invest in lenses and not bodies"