Humans behave strangely when they are involved or close to a dangerous situation. We did courses on it and one was a video of people waiting for what they thought was a job interview. A fire was remotely started in a bin. Everybody looked at it but for ages nobody did anything. Nobody even left the room. Eventually somebody made a move and that galvanised others into action. People are often in disbelief that it's happening to them so they kind of freeze. They look for somebody to do something, to tell them how they should act.
I also think these incidents seem to be happening so often people have become desensitised. Even if they've not been involved personally they've probably seen lots of videos of such incidents. So long as it's not happening to them they just act normally and hope it doesn't. They were on a train as well so you're often a bit limited as to where you can move to. If anybody is ever unfortunate enough to find themselves in a similar situation grab a fire extinguisher if you can, there are usually a fair few around on trains and in public places. A foam, dry powder or co2 one would be better than water. Spray in the person's face and once it runs out you can batter them with it. Pull/press the alarm too. Last resort obviously, the first plan is remove yourself from the danger.