Air Bag Not Deploy problems of the 2009 Audi A4

I had an accident in my Audi A4 convertible and the driver airbag did not deploy. Due to sun glare, I rear-ended a Mercury mountaineer with my 2006 Audi A4 convertible. I never saw his taillights, I was not messing with the radio or talking on the cell phone. I was not cited, although the accident was clearly my fault. I was the only one in the car. I was wearing my seatbelt and shoulder harness in the correct fashion. My injuries were minor bruising and a rub mark on my neck from the shoulder harness. The accident was a direct hit, practically straight on. The radiator was pushed into the engine. The passenger airbag deployed. The driver airbag did not. The rest of the Audi safety features worked - the rear seat rollover protection deployed. The emergency flashers came on. The engine turned itself off. My question is why didn't the driver airbag go off??? it was a direct hit. There was no one in the passenger seat, yet that airbag went off.