Montreal has a huge talent pool of qualified professionals that would have helped Amazon greatly had they considered the city. However, the bottom line is always business. Montreal just happens to be situated in the highest taxed jurisdiction in all of North America. I believe that the high tax situation was the main reason that the city was removed from the list.
So although the city has a lot going for it (entertainment, nightlife, low crime, good public transit...), business always comes first...always.