As David points out, the space under the platform is not necessarily safe. The trough between the tracks often has debris and/or some water in it, and appears quite narrow. To get in between the two sets of tracks involves stepping over the electrified third rail. None of these possible escapes would be inviting to someone in imminent mortal danger. Attempting to get back onto the platform would have been obvious split-second choice.
I heard Mr. Abbasi's explanation of inaction on the local news radio. He said he had defensively put his back up against a wall, as the pusher was running directly towards him. He seemed to indicate that he was operating the camera not to take pictures, but to cause the flash to repeatedly fire and alert the train's motorman of the situation.
Although young at the time, I recall the Kitty Genovese incident, which evolved over a period of time during which numerous people ignored cries for help, rather than make an anonymous phone call. Given our current culture, I'd like to think it wouldn't happen again, but then, I'd like to think I'd have helped Mr Han, rather than stand frozen, thinking, 'Somebody should help than man'. Who knows?