What was the reasoning behind the USA's policy of isolationism after WWI?
There were two main factors that brought about the United States’ policy of isolationism after World War I.
First, the people of the United States generally felt as if that war had not been in the national interest. They felt that there had been no good reason, in retrospect, for the US to participate in that war. They felt that the war had really been a European quarrel that was not motivated by any higher purposes than the desire for power. Some felt that the US involvement had been pushed on the country by rich merchants who profited from the war effort.
Second, and very relatedly, the people wished to avoid ever becoming involved in another such war. Since the war had been, in their minds, pointless, it was best to avoid involvement in another war. This was particularly true since the war had been so brutal and bloody.
The US, then, wanted to avoid getting pulled into another war that was A) terribly destructive and B) not really important to the interests of the United States.