The ruling elite (see bread and circus comments) are much more conservative than the general population, and so manipulate the system to their advantage. They are the only interest group, in the US or in any other country in the world, with the power, access, and wealth, to manipulate entire political systems to their advantage, whether they are doing it in Nigeria or Florida.

As NicoleC pointed out, Clinton was impeached not for what he did, but for who he was--a president who rose from the ranks of the American untouchable caste. He was not a Roosevelt, a Bush, a Kennedy, a Gore...

Carter was never successful as president for the same reason--he was a Washington outsider from humble beginnings.