You were limping. I got attacked in a park years ago by a man that was trying to rape me. A guy walked by and I called out for help, but he just nodded at the dude attacking me, as if he was simply having a problem with his woman. I escaped because I refused to simply give in, and fought with him.
Some people just don't want to be bothered.
However, if asked we'd all love to say that we would stop every time, but honestly; in this day and age, and with the fast pace and deadlines everyone lives with-is it truely possible to stop each and every time? I say no. It isn't. Regardless of your good intentions, things happen and emergencies come up, and you don't always have the time, or can afford to stop. In a perfect world, the answer would be yes, but we live in an imperfect world where ignorance runs rampant, and time has limitations.
I have stopped to help when and where I could, and that is the best that I or anyone can do; you assist the most when it is the most conducive to do so.