I think it is to do with having two pistons changing direction at the same time. With an even number of cylinders, they do, which would produce a more severe vibration. With pistons as large as the ones they used, this would have been a significant factor. This is the principle behind five cylinder car engines (Volvo et. al.).
The direction of rotation is governed by the inlet and outlet valve and ignition timing, and the starter usually gives the engine a clue
