Because the size of the bags were more easily adjusted when Canada switched over to the metric system. The bags are placed in an infinitely reusable container that holds the bag upright. You cut the front-facing corner of the bag, and you then have a simple way to pour it that will never be too much, so long as you cut the hole correctly. There's no need to reseal the bag once opened. They still keep long enough that it won't go bad before you finish it, unless you just don't drink milk often. Having the whole volume separated into three bags actually makes the keep time longer than that of a carton or jug, because you only open 1/3 as you require it.