Used to be, if a plane got hijacked, the passengers were compliant and waited for it to be over. United 93 changed that forever.

For a little more context, it was because the hijackers were holding it hostage until some demand was met, or they were flying it to Cuba or wherever, and then the plane and passengers would be released. On 9/11, they turned the planes into missiles with innocent passengers aboard, and they were all expected to die. When the passengers realized this, they took action.