Jimmy Carter's failed presidency comes down to the fact that he lost the faith of the American people and was ineffective at truly leading America. His personality led him to be in direct conflict with other politicians and centers of power in the government. Coming in as a Washington outsider, he promised big change and reform but was ultimately unable to deliver on it.

After the Nixon and Ford years, Americans came to view their government as being coldly pragmatic but, more importantly, corrupt and incompetent. Moreover, in terms of international affairs, the U.S. was encountering an international system that was becoming increasingly multi-polar. In other words, global power was shifting away from the two superpowers and disaggregating among the Third World states, Asia, and an increasingly integrated Europe. This disaggregation of power was most clearly symbolized by the U.S. defeat in Vietnam and a series of oil crises instigated by OPEC (a conglomerate of oil producing states based in the Middle East, in addition to Venezuela) that made gas prices soar in the U.S.

Carter believed that he could simultaneously renew America's trust in government and reassert America's leading role within global affairs. He failed in both regards.

Here's a deep dive on what led to Carter's presidency failure by Reddit user /u/gent2012: