When an emergency situation happens to someone in a public setting, the more people in the scene, the less the chance that any of them will act upon it by providing help or even call the authorities. This is called The Bystander Effect.
The reason is that everyone thinks "one of the other guys would do it". But that's not the case. If you see something, do something. Don't wait for others. People died because each of many people said "the others would do something".