Hocky is right, the sensors only measure the forward decelleration for front airbag deployment. Manufacturers try to make sure air bags go off only when they're needed... a rather daunting task. From what you described, most of the force was sideways in your episode. Your car's safety equipment performed as it should have.
BTW, lateral head injuries are more lethal than frontal head injuries - hence, the safety push for side curtain air bags. Consider yourself lucky.
95 E320 Cabriolet, 131K