What was it about Truman's background that helped make him a good president?

It is hard to say exactly. Truman showed leadership as an officer in World War I. He was a county judge under Pendergast machine in Kansas City and undoubtedly learned a lot about how to handle people and how politics worked. As a senator his Truman committee saved the country millions.

Truman was decisive. He thought he was right even when the matter was debatable. He understood party politics. He had the common touch. Particularly the rural people and the less educated people related to him. He was not prejudiced . He helped set up Israel as an independent country. He desegregated the military.