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:
The Carter presidency's reputation is worse than it should be. The truth is that Carter got a lot of things done as president while facing gargantuan problems that would overwhelm any president in recent memory. His problem was that he was fundamentally an uninspiring politician. In terms of his actual accomplishments as president, he got quite a bit done. Here's how Peter Baker at the New York Times puts it:
In Eizenstat’s rendering, Carter was a thoroughly decent, well-intentioned idealist who tackled tough issues like energy, wrestled with economic troubles and advanced human rights, all with drive and determination. He made peace between Israel and Egypt, ratified the Panama Canal treaty and started the military buildup in response to Soviet aggression that Reagan would accelerate. Despite punishing inflation and unemployment, economic growth under Carter was nearly as high as it was under Reagan, and he added less to the national debt as a percentage than either Reagan or the two Bushes.