Actually that guy did try enlist another person to help, but that person and everybody else ran, so buff ex-military guy followed. He did get back on the bus briefly later to see of he could help (quickly saw it was way too late) and was apparently involved in helping keep the attacker in the bus after the attack was over and before police arrived.
All it would have taken was 2-3 people to run headlong into the guy and lie/sit on top of him until they or someone else could get the knife out his hand or definitively disable him.
And while the first guy would probably get stabbed and maybe killed, the others behind him would likely survive.
So, if you want to be first, I'll follow right behind you.
By the time you figure out what is happening, that passenger may have been stabbed multiple times. You have to weigh the chance of saving them with the likelyhood you are going to get killed trying.
The main reason for intervening at that time would be to prevent anybody else from getting hurt. If everybody else is getting off the bus, and nobody is left to be killed, you would be risking your life to stop him after he probably already killed the guy.
So I don't fault anybody for fleeing. If he was trying to stab the bus driver, then I would have intervened, or if it was on an AIRPLANE where he might try to take over the plane.
BTW, an ex-military guy still might be no match against another man who has martial arts training and a knife.