Two reasons. First, so four people could experiece the ride together, instead of back-to-back seating that seperates riders. The primary reason is because of the way the forces are directed. The snap spin areas of an outer-facing seat force people into the restraint. The Gerstlauer version puts the force into a rider's back, which, at least on the computer mock-ups, led to a much more comfortable distribution of the g's.
Adam