They stayed neutral because of isolation. Some people felt like it was their war. So they became isolationists.
There was a strong type of opinion within the government as well as the people. Some may call it isolationism today, but it was basically that America shouldn't be everywhere, shouldn't run the world so to speak. There still was some geopolitical behavior going on it just wasn't as accepted or widespread as it is in modern times.
The United States stayed neutral because everyone considered it as "their war" everyone wanted to join and Americans didn't.