President Truman had an ambitious domestic agenda, which he called the "Fair Deal", which stalled in front of a Republican-controlled Congress. In the 1948 campaign, he ran against the "do-nothing Congress", which turned out to be a successful political strategy, although it didn't improve his legislative prospects. An image of ineffectual leadership is never good.
He also had an ambitious foreign policy agenda which was much more successful. This included the UN Charter, Universal Declaration of Human Rights, Berlin Airlift, Truman Doctrine, Marshall Plan, North Atlantic Treaty, and the Korean War. You have to remember that isolationist sentiment was still very strong in the United States. Truman's presidency was crucial because he kept the United States intimately involved in international relations in the post-World War II period. But a substantial faction of Republicans was opposed to some or all of these policies. In particular, the Korean War was carried out without a declaration of war, to enforce the UN Charter, under UN auspices, and with the authority of a UN Security Council resolution. Imagine what that must have meant to a country which 30 years earlier had rejected the Versailles Treaty and the League of Nations! And furthermore, the war turned into a stalemate, in which tens of thousands of American troops were killed. Dwight Eisenhower came into office largely on the back of this issue, and the idea that Truman was overextending the country. His corrective was to reign in the Korean War-era defense budget and put much more emphasis on nuclear deterrence, creating the Cold War as we know it. It was a sort of "return to normalcy" (to quote Warren Harding) that the American people were looking for. Eisenhower normalized the Cold War and had a simultaneously more unilateral and more restrained foreign policy.
One of the main reasons I can think of would be his relief of General Douglas MacArthur. MacArthur was a very popular general and national hero during WWII who helped turn the battle in the pacific around after being pushed out of the Philippians. He was also a distinguished soldier and leader during WWI. He was even considered as a political candidate during the 1940's elections.
However, during the push back against the North Korean invasion, MacArthur went against express orders in several respects and invaded the North, thus instigating Chinese involvement. Truman removed MacArthur from command in Korea thus receiving a huge political backlash of public support for the decision. He decided not to even run for reelection.
Casey, Steven (2008). Selling the Korean War: Propaganda, Politics and Public Opinion. Oxford: Oxford University Press.