I'd say that, as social politics have got more complicated, chivalry just isn't respected or appreciated by some people. Courtesy is often seen as a front for something else, perhaps an ulterior motive.
Also, many people enjoy discourteous, inconsiderate behaviour, so they're not going to appreciate acts of niceness.

But it's probably just related to the busier, more hurried, more frantic nature of city life these days. Thousands of people cross your pathe every day; no time and no point in having regard for faces you'll never encounter again.

I don't see that as an excuse for being discourteous, but it could be a reason...