This excellent summary of ideas on why America refused to join the League of Nations is from Ben Walsh's textbook, GCSE Modern World History (1996).  

Ben Walsh is a GCSE Assistant Examiner and a former Head of History.

Dislike of the Treaty of Versailles

The League was inextricably linked to the Treaty of Versailles.   Wilson had insisted that all the signatories to the Treaty should join the League.   The League was also supposed to enforce the Treaty.   Yet some Americans hated the Treaty itself.   Many Americans were recent immigrants.   There were millions of German immigrants who had never approved of America joining the war against Germany .   They certainly did not want America to prop up the League as it squeezed reparation payments out of Germany .   They wanted America to have no part in it.

Cost

Others were worried about the economic cost of joining the League.   They thought it would be as if the USA were signing a blank cheque.   The USA would promise to solve all international problems regardless of the cost.   Business leaders in particular argued that America had become a powerful country by isolationism - staying out of European affairs.   America should continue to mind its own business.

Isolationism

To many Americans the plans for the League of Nations suggested America was promising to send its troops to settle every little conflict around the world.   Americans had been appalled at the carnage of the First World War. T  hey wanted America to stay out of such disputes.

Dislike of the old Empires

Other Americans opposed the League because they were anti-British or anti-French.   They thought the League would be under the control of France and Britain .   Why should America get dragged into fighting for Britain 's Empire? Americans believed in freedom.   They opposed the whole idea of colonies and empires.   Surely America could not agree to safeguard all the colonial possessions of Britain and France !