Probably because (i) federal judges are appointed by the President, not elected by the people and because (ii) federal judges can serve for life.
There are three branches of the federal government: executive, legislative, and judicial. People vote for the President and members of Congress. After a certain number of years, the President and members of Congress must run for re-election. If they do not win re-election, they lose their positions. The President is also subject to a term limit: he or she cannot serve more than two terms in office.
The process to select and remove federal judges is different. The Constitution of the United States provides the President with the power to appoint federal judges, including Supreme Court judges, subject to approval from the Senate. Once appointed, federal judges generally cannot be removed from office until they retire or die. Because people do not vote for federal judges and there are generally no limits on how long they serve, we can argue that the judicial branch is the “least democratic” of the three branches of government.
Of course, because people elect the President and their Senators, voters can at least indirectly affect who is appointed to the Supreme Court and to the lower federal courts.