I think everyone on these threads misses some point of perspective.
In general, everyone wants successful, powerful people to be nice to them. They don't really care about unsuccessful, weak people being nice to them. They don't object but it doesn't make them think better of those people. Why is this? Because powerful people can do things for you whereas weak people can't. Having a powerful friend is more useful than a weak friend. Therefore people tend to be nicer to people more powerful to them, and not as nice to people less powerful than them. This is despite the fact that more powerful people don't need to be as nice in return, and less powerful people will likely be nice to them anyway; the relationships aren't reciprocal.
I use "power" in a very general sense here, just to mean they can give you something that is valuable to you: a good grade, a job, a desirable relationship, whatever. These relations are more usually trades than gifts, so two powerful people are more likely to come to an agreement than a weak person and a powerful person (a good employee and a good job for instance, rather than a bad employee and a good job). Similarly, both people in a relationship want to enjoy it so usually relationships are between people of comparable desirability.
People who are nice to almost everyone usually act like that because they have a weak position. They're not that desirable and can't do much for others. Others therefore don't tend to treat them that well and don't tend to give them things that they want, but this isn't because they're nice to everyone. That is confusing cause with effect. If they were arrogant and rude to everyone they would be treated even worse, first for being arrogant and rude and then again for not knowing their place. Their niceness to everyone is a reaction to their weakness, not the source of it.
A "nice guy" who was also extremely attractive, charismatic and successful would be very popular with everyone. On the other hand a guy like that wouldn't need to be nice to everyone to get what he wants, which is why it's comparatively rare.