It looks like they are reusing the power brick design for the laptops. Plus the prong plug is swappable with a longer extension or international adapters. It would be akward for the swappable piece to be on the broad flat side, since they have so elegantly engineered the little corner swapping thingy.
But it would make sense to make it flatter for wall use. I would say they should have hinged the prongs so that they lock into place when the unit hangs via gravity. It looks to me like the prongs would want to rotate with the weight of the unit.
A more cubic design would be more balenced, but this "deck of cards" shape is more conducive to putting into a laptop bag next to the power adapter.