It is mostly hindsight and machismo, though. We know now that these people had nothing to lose by fighting, but the reality is that this was probably not clear at the time. Hijackings have historically not necessarily been grave life-or-death situations, and flight crews are no doubt trained to realize this and try to encourage people to not do things that would be likely to make the situation worse. For example, there have already been 5 hijackings worldwide this year (before today), 7 in 2000, 8 in 1999, and 14 in 1998 -- only 4 of those 34 flights resulted in fatalities, 8 total out of thousands of people involved.

What happened was horrible and unprecedented; it is very difficult to go back and speculate what any of us would have done. It doesn't matter, anyway.