Focus on the same reason they lost the Vietnam war despite winning many battles. The US people may not have the will or stomach for a war when a threat isn't staring them in the face. It's really the same case with most nations, it's why pound for pound the US is so inefficient in warfare historically compared to many others.
The old world had the "benefit" of facing an imminent, existential threat during their wars up to WW2. The less tangible factors like morale and willingness to fight and die for your homeland are much easier to embrace and support when armies are literally marching through your backyard, when you can't go to work because the factory you've made your living at is a target of a bombing run, and when you're on the wrong end of the barrel of a gun. The average US citizen at this time had none of these factors to worry about, so selling them on fighting a war that only might venture across the Atlantic and visit them is tough. There's a non-insignificant faction of people who would likely just see it as a waste of life and resources that would be better spent improving conditions at home.
The leadership of the US might be interested in pursuing long-term goals by intervening in the war in Europe, but John Q Public really has no desire to submit to a draft and face the possibility of not coming home. It took heavy propaganda efforts, censorship, and a Japanese attack on a US military base to actively pull them into it, even if Washington was providing materiel support to friendly nations throughout the world, and even then the US was limited from bringing its full corpus to bear.