This sounds like the sort of thing that has a specific answer in your book. You might want to be sure to look there because we might not interpret this question the same way your book does.
The most plausible answer to this is that the major ecological zones or landforms in North America run north to south while Canada (especially the part that is within the temperate regions) runs east to west. For example, the Rocky Mountains are a major north-south mountain range. As another example, the Mississippi-Missouri River complex runs north-south. Even the mountains in the east (which are very old and are no longer very high) run north to south. The fact that these landforms run north-south makes it more "natural" to have a country that runs in that direction or that at least (like the United States) has a much longer north-south dimension within the temperate zone. By contrast, Canada's main axis is an east-west one in the southern part of the country.