Because that was the only remotely appropriate gearbox we had in inventory ! We hoped we could get away with a single FP or 550 size motor to minimize high up weight. We had hoped to reduce chain related losses (and sprocket & chain weights) by direct driving (through the AM). We also had no idea what sort of drop in RPM a largely unloaded motor would incur trying to shoot the ball (drop in RPM means drop in exit velocity of the ball relative to the no-load rpm). Heh I'm not in mechanical
But then we found the recover time slower than we would like. Shooting distance was limited to 14 ft which some team members wanted more so we put on a more powerful motor. Unfortunately its KV was higher.