Answer: Because of the Atlantic and Pacific ocean
Explanation: The map provided below shows the United States allies during world war 2. The blue is the US allies and the Black is its enemy's. Looking at the map you can see that there are no enemy's present on the continent of North or South America. The United States was "Virtually" Protected by the Atlantic and Pacific ocean during world war 2. All the fighting was on the continent of Europe and Asia, thousands of miles away from the US mainland's. The Large ocean and the US large navy kept any attempts of its enemy's attempting to attack its mainland's all failed during world war. The US mainland was logistically to far for Germany or japan to effectively attack the united states. The technology during worldwar 2 was very limited for war ships or planes to be able to go at that long distance.
Look at the picture below for a better understandings