I always chalked up the half octagon design popularity due to some common 4 player designs. A square control panel would jab into player 3 and 4's gut when 4 people are crowded around a machine. The controls were not diagonal, but it allowed players to not have to stand 90 degrees to the side.
Isn't that half Hexagon and not Octagon? Forgive me being a geometry nazi :p
I even put a slight tilt to mine so it would be ergonomic. :cheers: