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.