A previous poster mentioned 'mob mentality', it's not quite that but 'bystander effect' is a recognised social psychological phenomenon.
Basically that when in a group, people recognise that something is wrong or needs to be done but the fact of being in a group sends them into apathy as everyone expects someone else to do something and therefore no-one does.
It was researched and recognised after a woman was murdered in public with numerous witnesses but no-one did anything because it was so public that they all thought somebody else would do something.
That's an extreme example but it explains the phenomenon sometimes. Most people aren't sat there not giving a shit, they're thinking 'I'd do it but my x problem or x worry is too great and that person over there looks healthier than me so they should or what if I do something and people think I'm interfering or weird etc'.
I've been at a festival where someone got seriously assaulted, blood everywhere etc and the crowd just moved away even after the perpetrator ran off. I heard his partner screaming and wandered over to find a barely concious man on the floor. I helped and directed the partner where to run to get more help but I'm a nurse and it goes against all of my personal and professional instincts not to.
I don't think most other people in the crowd didn't give a shit or didn't care, I think they thought 'someone else, probably betters at this than I am will come along in a minute'.
I think most people ignoring a pregnant woman in distress or worse (as a PP said and how awful to have experienced that btw) fainting on a train/bus are thinking 'someone else better than me will be along in a sec and solve this'.