Becuase the average human is inherently cowardly, and has never seriously contemplated thier own demise, any more than most livestock has. Something scares them, and they freeze, on the theory that if they don't move they won't be seen. Then greed kicks in, they pray that will be saved, the beg, they cry, they try to run away. They'll even release wastes, hoping to disgust the predator.

They are also inherently greedy and ungrateful, due to their upbringing, social dynamic, and age. To them, to be injured period is an alien concept. To injured by another, shockingly so. And being injured or killed to protect someone whom you don't have a biologically-programmed instinct to protect just doesn't even enter into most of their minds. Ever.

What happened on Flight 93 is that they had time for thier thinking brains to kick in.