They say that the League of Nations was the work of President Woodrow Wilson, but America never got the chance to participate here because of different reasons. The first reason is there were so many casualties during the war that were civilians, and they felt that participating will be harder on the nation as a whole.
They also believe the joining will means that there will also be setbacks to the current economy of the USA at that time. Another thing is the country believed that they would not get as much support as they have expected. In short, America felt that they had had enough of wars, and they do not want to add another war anymore.