There was a young, buff ex-military guy sitting right in front of the attacker and the victim. He ran.
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.