I think it all comes down to time. Everyone in NYC is working, on the way to work or on the way home from work.
So, people in NYC are busier than the avg person in the USA. We don't have the time to stop and chat/etc. There is also the anonymity factor to consider also...in a small town everyone knows everyone, so being rude to someone will have consequences later....obviously not the case in NYC where people know they will never see that person again.