Pizza boxes are usually a single piece of "card" and the flat shape (web) often pressed out using a form template, similar to a deep cookie cutter. They can sometimes have liners attached or cut-outs or shapes pressed in.
Its easier and cheaper and uses less complex and expensive equipment to crease straight lines for folding than curved ones.
The "square" box is also usually more straight forward to work with during the design and print stages, depending on a number of factors.
The amount of difference in materials is negligible and more likely more used for the squared than round box. Waste is reduced at manufacturing end.
"Round" boxes would be more likely to be octagonal (for creasing) or have to be manufactured a far more expensive way than "square" ones.
There are exceptions to the above though but basically "square" is more straight forward than "round" during all steps from design through manufacturing then store staff through to end-user.
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