Why did the US want to stay isolated at the beginning of World War 2?

Because the US had already fought WW1 in Europe only 20 years earlier. The US thought that the Europeans had learned how to manage their governments by this time.

The US was also trying to respond economically to the Japanese expansion in China and the South Pacific (which was what eventually brought the country into the war following Pearl Harbor). This concern also added to the reluctance to commit US troops to Europe.