During WWII, Why did the United States withdraw from world affairs?

If I understand your question, you're alluding to the fact that the US did not enter the war until December 1941, two years and three months after the war in Europe began. The reasons for this are simple, and can be summarized as follows:
Part of the rationale for the independence of the US has always been to create a haven for those who wished to escape the perpetual wars, violence, and conflicts of the Old World. Washington's warning to "avoid foreign entanglements" encapsulates this idea. It was never possible for Americans to remain totally aloof from the rest of the world, but the goal of doing so has always been, and continues to be, an ideal for many Americans. However, as the US became a major power in its own right, it was inevitable that it would be drawn into international conflicts. The entry into World War I by the U.S. in 1917 was delayed until nearly three years had gone by of the four years and three months the war was to last. After the massive and unprecedented carnage of World War I, there was strong pacifist sentiment among all the victorious powers: Britain, France, and the US. The British and French did not really want to stand up to Hitler, but finally did so in 1939 when the Germans invaded Poland. Given that the European governments wished to avoid war as long as they could, even when the aggressive actions of the Nazis were taking place in their own backyard, to put it colloquially, it's hardly surprising the US wanted to stay out of the conflict.
Apart from the American wish to avoid involvement in any war, there was an element of xenophobia in the US, especially within the "America First" movement. It was unfortunate, and it's embarrassing to have to acknowledge this today, that in both Britain and America there were many who had a sneaking sympathy for Hitler. As influential a person as the famous aviator Charles Lindbergh was openly anti-semitic and asserted that the US should not get mixed up in a war which, he claimed, was being fought "for the Jews." Given all these circumstances, and in spite of FDR's wishing to side with Britain in the conflict, there was insufficient support in Congress for participation in the war until US territory was attacked by the Japanese in December, 1941. Had this not occurred, it's anybody's guess when or even if the US would have relinquished its isolationist stance.