Reinforced doors and a belief among everyone from passengers to pilots that a successful hijacking is likely to lead to death rather than inconvenience reduced hijacking after 2001. I would be surprised if searching passengers more aggressively or requiring people to prove their address to get a driver's license had much to do with it.

Yeah, I think passenger game-theory is the number one reason, and that was already in place on September 12th, 2001.

Passengers are now aware that hijackers might actually just buying time until they can trigger a murder-suicide attempt, and many will believe it is likely enough that they need to fight for their own lives. The old assumptions that "almost everyone gets out alive by passively cooperating" no longer hold.