I am from the USA so maybe a six pack is not a universal standard for beer grouping everywhere.
You have a pretty good answer here.
As a bit of foreign comparison, beer for the longest time in Egypt was sold in cartons of glass bottles. You could choose big bottles or small bottles. The big were 750 ml. Returning the glass bottles was important to the breweries, especially after 1950, when the local glass bottle became inconsistent and bottling materials had to be imported to work with the machines that were brought by Heineken. Cans did not really enter the market till the late 80s early 90s.