My guess is that the round exterior and by default, the round interior where the beds are is stronger than a squared off section would be. I remember the geodesic domes that were strong due to their shape. If it was square I would guess again that there would be more stresses on the corner. The round shape I believe is stronger. It's all a guess on my part. Maybe someone with a structural engineering background could verify yay or nay. More room on the bed in a rectangular shape would be nice, but I make it work for my feet facing aft.
John