[Robin_Message]

Bystander effect (http://en.wikipedia.org/wiki/Bystander_effect)—since no-one else is helping, you don't want to, and since there are all these other people here, maybe they should be the ones to help. Maybe one of them is better trained to help than you.

Inconvenience—trivial sure, but can easily be rationalised as "who wants to be late to work in this climate?"

And personally I'd be worried about offering to help a random, confused, nervous looking person with blood on them. I would like to think I would help, but it would worry me.