"The more other people are around, the less likely we are to help a victim."
As with Groupthink we allow the actions of others to serve as research. If a bunch of other people are doing nothing, well, they must know more than I do.
Additionally, action & inaction are exposed to asymetrical risks. If a man does nothing he is one of many. If he acts he stands or falls alone.
The fear of embarrassment may be a consideration as well. Group inaction may set a social norm for the setting.