The term limits of the elected offices of the United States were designed to represent the amount of popular pressure the officials were intended to respond to.

For example, the House of Representatives has an election once every two years. This rapid, direct, election means that Representatives must constantly legislate with an eye to popular opinion, as another election was only ever 2 years away. Their policy should represent the people who elected them. On the other hand, Senators serve for 6 years terms, and were elected by the state governments until 1913. Thus, Senators would have a long period to legislate, independent of popular concerns or fad-ish political movements. Only once every 6 years did Senators have to concern themselves with an election, and as I said, it was not strictly a popular election.

The Supreme Court then, was designed to remain independent of popular opinion and contemporary politics. They were supposed to legislate only on the basis of legality (because remember, its not until 1803 that the Court gains the ability to judge constitutionality. Before then, it was only a high level court.) Judge were supposed to be impartial to political pressure, popular pressure, or domestic movements.

The 1803 decision Marbury v. Madison made everything weird. While judges since are supposed to only support the strict wording of the Constitution, interpretations are hard to standardize. Many judges, both intentionally and unintentionally, have made decisions which were influenced by their politics. The Court today does not work in the way in which it was first designed, so the policy-making power of the Judges seems to be at odds with notions of democracy.