Even prior to 9/11, not many planes were hijacked in the US anyhow. The only notable one on that list in the 90s was a FedEX plane. 9/11 changed peoples perceptions due to the terrible events that followed the hijackings, but even then the idea of using a commercial plane as a weapon wasn't new. It seems Air France Flight 8969 in 1994 was the first attempt, though it failed.