As I wrote in detail in this thread:
https://www.canonrumors.com/forum/threads/my-experience-from-canon-5d4-to-nikon-d850.37563/ I moved to Nikon F (just over a year ago) after twenty years with Canon. The D850 is the best DSLR ever made (my opinion, but also the opinion of several others). I love my 500PF lens, which no one else matches. It is also worth noting Nikon still makes and sells a (very expensive) 35mm film camera, which Canon has completely abandoned.
So why does Canon have a huge lead in sales? I will speculate it is due to the following...
1) Canon customer service and repair service is noticeably better than Nikon.
2) Canon has better marketing that includes sponsorship of big name pro shooters.
3) Canon has had a simple and consistent roadmap of lenses, meaning (as far as I know) every EF lens of every generation works with every EOS camera of every generation. Nikon has a confusing roadmap where generations change and lenses from certain eras do not work with cameras from certain eras, older lenses have physical aperture rings while newer ones don't, etc.
4) The 500PF lens, which is in huge demand and might have pushed some new users to Nikon, is perpetually out of stock to this day and always has been. I lucked out getting one quickly, but customers are on a waiting list up to eight months to get one.
5) Canon got an early lead in digital, with the 5D series and 1D series large megapixel sensors, when Nikon was stuck at 12MP for years. Many people jumped to Canon in the early days of digital and once they are invested in a system it's hard to win them back.