However, further to Professor Latane's opinion, I would say that the resistance to actively help someone when others are around is because of a fear of how those others will react - in short, we don't feel we can rely on the goodwill of our fellow citizens to back us up. (If I had stopped to help, who would have stopped for me when the knife was turned in my direction?).
The sadness is that apathy is infectious. A decade of selfish economics, a criminal justice system that fails to protect victims and witnesses, and pressure groups that constantly harp on about individual rights have all contributed to the present mindset, which fails to recognise that as individuals we also have responsibilities.