This is an extremely nuanced question. I did part of my thesis in grad school on why the Treaty of Paris was such a failure long term. I can speak to a few of the reasons why the USA did not join. I am by far not an expert all aspects; I have a couple book recommendations down below that do a much better job explaining the treaty and Wilson's objectives.

First, we need to remember the context of what is going on in post-war America. America in 1919 was full of workers strikes, the flu, various forms of corruption. The war was not 100% popular in the first place. It is not as if the US went to war and everyone decided to be supportive. There was partisan politics in play as well, which I will get to. Essentially, the war ended and for the general public it was time to go back to the status quo. There was a lot more going on at home that needed our attention. Europe may as well have been another planet. It also does not help that many Americans were aware of anti-immigrant sentiment and were doing their best to distance themselves from the Old World and be more "American". That will come into play in point three.

Second, Wilson went to Europe. Which in hindsight was a horrible idea. It's a long boat ride and he spent not an insignificant amount of time away from DC. This allowed his critics to have complete access to the newspapers and to form the narrative of going back to more isolated America doctrine. Basically, if Wilson was for something, they were against it. Anything to undermine Wilson, which is not unusual for DC politics. What was unusual was the President not actually in town to play offense. This is long before the time of a massive executive branch that could speak for the President. Also, his biggest critic was an ex-president.

Which brings me to my third point and more my wheelhouse which is former President Theordore Roosevelt. President Roosevelt was still around politically, even after his loss in the election. He a lot of opinions and some believe was still interested in running again. He was a massive celebrity and beloved in certain corners of the country. My specific research was focused on how much influence he had in the newspapers. If he gave a speech or he stated an opinion, it was printed. He was strongly anti-German and was certainly not going to do Wilson any favors. He took full advantage of Wilson being distracted and was using it to shore up his base of support. He led a movement to eliminate German language from the schools and in media. How did this influence policy? It forced German-Americans towards proving their loyalty to the country. Doubling down on focusing on domestic politics and ignoring the European troubles. It was not universal obviously, but it was yet another part of the country working against any idea of being involved in foreign entanglements.

I am sure others can add tons to why. However, you cannot discount how out-matched Wilson was in general as a politician. There are some good memoirs out there regarding what happened during the Treaty of Paris. The narrative was Wilson was outmaneuvered at every step by his foreign counterparts. It was no different at home, only he was not around to challenge his DC rivals. There is a lot more to the story, it can be argued though that public opinion was soured on the treaty (and thus the league of nations) before it even came back to the US for debate. By the time the treaty was signed and he had to turn his attention in convincing Congress to pass it, it was too late. The narrative was written and America as a whole was consumed with too many domestic issues to make it a high priority.

I am bias, but the entire time period post WWI is fascinating study, there is a lot going on and even my short answer is just a tiny piece of the entire story. I could go on for a bit on how unbiased the newspapers were at the time. How much influence they wielded in turning the public against any concept of the US being a world partner.

I wish this could be more detailed, but I'm going a bit off memory of something I did a decade or so ago.

I have not read it in a long time, but I recommend To End All Wars by Thomas Knock. It is a great book related specifically to the Wilson and what he wanted to accomplish post-WWI.

Also, Paris 1919 by Margaret McMillan is a nice summary of what was going on during the negotiations.