Canada's southern border is huge and almost its entire population lives very close to it. The USA could occupy our major cities very quickly and our military is far from a match for theirs. The whole system has been designed on the assumption that the USA will not become an enemy and can be reasonably expected to help defend Canada from other enemies, and Canada can manage with relatively small forces since it's geographically quite well protected except on the southern border. It was not designed to anticipate a rapid takeover of the USA by Nazis who immediately decide to attack all their former allies. We wrongly assumed the USA had its shit together even just a little bit.