Well, as a Korean, I think that this is due to the difference of importance of how other people think me. I've heard that the western world is very individualistic, and people in general don't care much about how other people acts.

In Korea (and AFAIK Japan too), people very care of what other people think about them. They consider the consequences of every action carefully. One of the results is that people generally don't litter in front of other people, and that makes the city clean. The places without much people, they are dirtier than others because there aren't other people around them.