Earlier quoted context omitted.
Shipping durability (plastic instead of a cardboard carton) and ease of machine handling ("sqround" instead of square). Both are covered in the blog.
They never really talk about alternatives to the plastic bottle, just variations of it. I would also think filling and distribution of a carton is a solved problem considering how much milk/juice gets moved in them without too much damage.
They talked about shipping durability, which was the point I was making. A cylindrical-ish plastic bottle would be less prone to crushing than a square cardboard carton.