Henry Cabot Lodge's Objections to Article 10 of the Treaty of Versailles (1919)
Why do you think the Senate was more receptive to these arguments in 1919 than in 1945, when the United States joined the United Nations?
I would say that experience and time would be the major reasons why the Senate sided with Lodge in 1919, but didn't buy his reasoning in 1945. At the conclusion of the First World War, Americans were still leery of Europe. The fact that so much in way of secret alliances, prolonged hostilities, and intense battles were fought with so much intensity caused a great deal of fear in the Americans. At the same time, I think that a level of mistrust was evident. When Wilson goes to the peace conference at Versaiiles and favors less stringent sanctions against Germany and yet is outvoted by France and England, Americans were able to see the level to which vendettas could be pursued in Europe. To become embroiled in this through a League of Nations was something from which Americans were ready to flee. Infact, the isolationism that began to become apparent in America throughout the 1920s and 1930s could be traced to American discouragement of the League of Nations. This perception changed after the Second World War. Americans were convinced that they and the Russians were the only powers left. Europe was crippled by two World Wars and the dropping of the Atomic Bomb on Japan led America to believe that it was one of two lone superpowers. The combination of a fear of the Soviet Union as well as a belief that isolationism would not stop the spread of Communism and other threats led them to embrace a United Nations. It should also be noted that the construction of the United Nations was nowhere near as binding than the League of Nations, for as a member of the Security Council, autonomy and freedom was still intact, something that Lodge was primarily afraid of losing.
To me, the major difference here is that, in 1945, the US was essentially in control of the making of the United Nations and could set it up as they wanted. In addition, I think that they used this power to set the UN up in a way that would not have the same "problems" that the League of Nations had.
Specifically, the UN was set up so that the five permanent members of the Security Council each had a veto on any action taken by that Council. This meant that any of the five countries (including the US) could veto a military action by the Security Council. That way, the US could not be dragged into a war against its will in the way that it might have been under the League of Nations.