Proximity: The US and Canada are very close together. I have forgotten the percent but it is over 80% of Canada's population lives very close to the Canadian/US border. That makes it easy to reach Canadian markets with US goods, and certainly US markets with Canadian goods.
The Border: Up until 9/11 the Canadian/US border was a formality. People crossed on easily from one side to the other. It made commerce one of the easiest in the world.
Language: for the most part especially in the west, the language spoken is about the same of either side of the border. The east is a little harder because Quebec is dominated by French as a language, but money conquers all.