If you replace "Western" with USA your statement is more accurate. Bag of milk can be found in western countries like Canada, Irreal, and generally in continental Europe.
Edit: capri sun juice pouches!
Also, Caprisun pouches have foil backing that cause them to somewhat maintain shape, and a design that gives them a base to stand upright. Canadian bagged milk, at least what kind I encountered when I was there, had neither of these traits.
Of the 5 provinces that still have bagged milk, Ontario and Quebec constitute over half of the Canadian population, so I don't think it is correct to say it's "not very common in Canada" when 26 million of the 40 million Canadians have access to bagged milk.