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
becasue woodwrow Wilson did not believe it would help the USA if it was to join the league of nations. Its questionable however that woodwrow was out to fool the world because he made the majority of the policies, this was never prooved but what 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