Hydraulic Power Assist Steering System problems of the 2001 Jeep Wrangler

My wife lost control of vehicle on city streets. She was going approximately 20 - 25 mph. She went into a resident yard where she hit a 8 to 10" dia. Tree and went over it. She then continued across side street and into another yard where she came to a stop against another tree approx- 6 to 8" dia. The seat belt appears to have worked as expected but the airbag did not deploy. She hit her lip on the steering wheel and required many stitches to repair it. She also had large contusions on both of her shins. The bumper was pushed into the radiator. The grill was pushed into the radiator. Both front fenders were buckled as was the hood. Why didn't the airbag deploy?.