Well, I will take a stab at this:
1) the offending aircraft were commercial US airplanes. NORAD was geared up for externally sourced threats
2)the use of the planes as missiles was a new, unprecedented appoach to terrorism
3) The hijackings were presumed to be of the same nature as all previous hijackings, transport to politically safe places and/or free advertising for the hijacker's complaints
4) All attacks were simulutaneous and again, unprecedented
5) It won't happen again, as evidenced by the fighter scrambles and escorts of any aircraft of dubious status.