Several Reasons.

Isolationism: At the time the US was still fairly isolationist. For a quick example congress voted down the League of Nations against Woodrow Wilson's wishes.

Several prominent Factions and people wanted to aviod entanglements. Henry Ford, Charles Lindbergh and Father Coughlin where a few.

German Bund: The German Bund was a pro German group in the US that pushed for the US to remain neutral during the war. At their height they pulled 20,000 people into Madison Square Garden for a rally.

Preperation: The US was not prepared for war and could be argued the the US was "barely" ready by 1941 when it finally got pulled in. The US did not have a decent Navy Fighter till a year after the war started. It did not have a decent long range fighter escort till later. It did not have a decent tank till 1942. It did not have nearly enough carriers, battleships, cruisers, and destroyers till 1942.

Additional to those below, there was a strong feeling in the US that they had been dragged into the First World War by British propaganda, and that the war had only brought problems. This strengthened the hand of the pro-German lobby throughout the 1930s, who echoed the sympathy toward Hitler's nationalism that was common in many nations.

Cynically speaking, the Germans had defaulted on a large volume of US loans in the early 30s, after Hitler came to power. This led to Britain becoming a large debtor nation, especially so after war was declared and the British brought supplies and weapons without restraint. If Britain fell, these loans would never be repayed and the entire path of American prosperity after 1945 may be questioned.