I think the point is that even in Canada, etc, it is still only the milk that is bagged. Not other things like orange juice. But I’ve never been to Canada so I dunno
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.