Why?
Maybe because rectangular are cheaper and easier to mount solidly, and (arguably) they would catch less wind.
Maybe because wiring 36 in series, with a center tap for bypass diodes, just makes more sense in 4x9.
Maybe something more scientific, like growing the crystals in proportions needed for a square 225W class panel would be problematic.
Maybe something more simple, like "it fits on a skid".
There are / were some nearly square units around. They were in the 30 to 60W class. I had some BPs or GEs a few years ago. I saw some generic Chinees ones later.
The larger units were a PITA to ship in quantity... It was cheaper to ship individual boxes of 4 panels via UPS, than ship skids via FOB.
G-