I usually don't try and speculate, but in this case I will. I think either they were extremely engaged or fell asleep. I have flown the A320 and with moving maps, selcal from dispatch, etc., I don't know how they could miss the airport by that much. It seems very difficult that they would not have noticed. That said, most of us that have flown for awhile have flown tired. Many of us have nodded off, and it isn't a good feeling to wake up and find you are the only one awake. If this was the case with these guys, it can happen to anyone. I hope this incident keeps the heat on the FAA to establish new flight time limits.