The United States never joined the League of Nations as a result of the decision by the US Congress not to ratify the Versailles Treaty. This refusal to join the league also fitted in with America's desire to have an isolationist policy throughout the world.
The U.S. before World War 1, and immediately after did not want to be involved in European problems. Many still felt we could be isolated from the problems of the world, so they did not want to join a union of foreign governments. answers.yahoo.com
President Wilson had just had a stroke-so his 14 points were ignored by the Europeans. There was great sympathy for Germany in Congress and a fear that the punitive damages insisted by the French would collapse Germany's economy (which it did) The wiki.answers.com
Some reasons: The United States didn't join. The countries of Europe were bitterly divided after World War I and were in no mood to establish a substantive organization dedicated to promoting world peace and understanding. The US being an outsider www.quora.com