The only real answer to this is that the US entered WWII because the Japanese attacked the US naval base at Pearl Harbor. The US declared war against Japan because of that action.
Many in the US government, most notably President Franklin D. Roosevelt, had wanted to get into World War II before the Japanese attack. However, there had been no way to do so given that the mood in the country was very much against entering the war. That mood changed immediately, however, once Japan attacked Pearl Harbor. Americans wanted revenge then and there was no essentially no opposition to going to war. This was true a few days later when the US declared war on Germany in response to Germany's declaration of war on the US.
So, the US may have had other reasons for wanting to go to war, but the Pearl Harbor attack was what actually caused the country to enter the war.