It is hard to manufacture a round box. Boxes are made by folding. Folds and bends are made with a straight line. A round box could theoretically be made but it would have to be a material other than corrugate. It would have to be a material that could expand into a mold.
Corrugate is cheap and the boxes are easy to make.
I trust it is due to some kind of Obama / Islamic conspiracy to take us away from the type of traditional circle-based pizza containers used in Biblical times, to a more socialist/jihadist square format.
because it's called a pizza "box", therefore it should have angles. the "boxes" for slices are triangular. besides, if you don't want your pizza cut at the store, the shape of a square makes it easier to cut at home. just try doing that with a round container.
I personally think it is ridiculous that it's not allowed. But according to polls the majority disagree with me! Surely there must be somebody on here that favors the outlawing of marijuana. Tell me why!