80% of professional photographers use Canon, Why?

It's difficult to remember that far back, but the Canon EOS cameras of the late 80s were simply much, much better than the equivalent models from Minolta (the leaders in AF SLR's up until then), Nikon & Pentax. Canon made a number of design decisions that turned out to be soundly-based: a) use a new lens mount with all-digital communication; b) put the actuators (motors) for moving parts, e.g. lens focusing, diaphragms, near to the part that has to move, don't do it via long mechanical linkages; c) USM motors; d) the combination of 'button & dial' to vary settings - it was new then. And of course in 1988 Canon had all this patented very tightly; it took a number of years for competitors to find ways round the patents.

The original EOS 650 / 620 were good cameras but not aimed at pros; the 600 (630 in USA) was even better; but the first EOS 1 (1991?) was a truly awesome camera. It had all the EOS design advantages, superior AF performance, and a comprehensive range of lenses & accessories. At that tme it was simply better than the competition, and the pro market, especially the newspaper & sports pros, turned to it almost universally.

Nikon may indeed be winning back some ground - the D2X was a very important camera, with the D200 as a backup, and the D3 / D300 combo is doing even better - but there is a long tradition now in professional shops of 'use Canon', and it will take more time to overcome it.

I'm not going to argue that these days any particular camera is especially better than any other, in the same market - the D3 is excellent but there again so is the EOS1DsIII. But between 1991 and the end of the 90s, the successive 35mm EOS1 cameras simply were the best, and their market responded.