What I've heard of the incident on the radio is that
A: Mr. Han was intoxicated
B: he and the man who pushed him were involved in a loud altercation with profanities exchanged
C: Mr. Han MAY have been partially responsible for initiating the altercation (this in no way justifies what happened, but merely goes to the state of mind of any witnesses)

Given these circumstances, were I a witness, my instinct would be to A: get away from those arguing before the push occurred, and B: look for a stationmaster or transit cop to help or to try and signal the train. trying to lift someone drunk and belligerent up off the tracks is risking being pulled down yourself. And I don't blame people for being too far away to help. The whole thing transpired in a span of less than 20 seconds. As someone who rides public transit here in DC, my instinct be it on the platform, in a bus, or on the station platform is to move away from anyone exhibiting drunken and/or violent behavior, as quickly as possible, so hopefully there is no incident to begin with. I can better help someone by remaining safe myself and then calling and/or directing first responders than if I step in to a fight. I think that's true for most people.