Well, comparing to other western countries, you could say that for a developed nation rich people have a lot of power in the US. Now it's a large country, so therefore the government has to be bigger, and that makes corruption easier to get away with. There's not really very much you can do about that, but combine that with the fact that the US economy is very much reliant on privately owned... well almost everything really, and ways for companies to influence votes without explicit bribing (like offering senators jobs at their company, giving them large salaries for a job with no real purpose), being legal, and you've got lots of oppertunities for the rich to steer policies and laws in the direction that is most beneficial to them, making the gap between the upper class and middle/lower class even bigger.
A perfect example of this is the net neutrality drama a good few months back. The vast majority of the US population was against repealing net neutrality, yet because a few people responsible for leading the FCC had close ties to internet providers, the vote went against net neutrality.