In order to understand this decision by the United States, it is first important to understand the foreign policy of the time and the lack of support for the war by the American public. The world was only 20 years removed from World War I which shocked Americans to the realities of war and the brutality of trench warfare. In general, the American public was not ready for another major conflict after World War I stayed for so long as a stalemate and the cost of living that the First World War caused. The United States in 1939 was not the military powerhouse it is today and it was a considerable risk to the United States to participate in another worldwide conflict.
As well, many Americans viewed the development of dictators in Europe following World War I as a sign that World War I ended in failure and many wanted to remain separate as to not get pulled into another major conflict. In fact, many Americans viewed World War II as a European problem and did not necessarily see America as having a large role to play. Also, America didn't enter WWII until the bombing of the Japanese at Pearl Harbor.