There was no good reason, neither would there be for the United States to have joined the war before they actually have joined in current timeline for a number of reasons:
1) Mixed Population: Failure to provide a adequate reason for joining one or the other side would result in mass rioting in the best cases or even coherent groups of guerrillas who would fight in the home-front against the U.S. government. By addressing the U.S. as a mixed population, we must remember and understand that the U.S. has always been the land of immigrants. While they do "give up" their claims to their homeland, it has been proven again and again that many immigrants brings their memories, traditions, and beliefs from their homeland. This entails for different people groups to suggest that the U.S. help their own homeland, in which, as a land of immigrant, you cannot satisfy everybody. With the attack on Pearl Harbor as well as Germany's declaration of war on the U.S., it simplified the US's choice in joining their long-time ally of Great Britain.
2) Military Population: This can be addressed with the terms of "Military Power". The U.S. standing army was small during this time, and the military hardware were older. While they were able to provide many equipment to the Allies through the lend-lease act, the type of equipment provided suggests the state of the U.S. military production. What was created generally were liberty ships for the navy, in which, while they were created as transport ships, failed to adequately address the defensive needs of a ship in a hazardous zone. The U.S. also failed to produce adequate tanks, with many taking the liking's of 37 mm guns (hardly up-to-date), and failing to maintain tank designs, leading to the creation of the beast of the M3 Lee (which had it's main gun added into the hull, which is a failure as a medium tank). Of course, the problems were addressed with the M4 Sherman, which, while it had addressed many problems, and was seen as a success, did not come with it's own share of problems. Granted, the M4 was an extremely well-designed and well-made mass produced tank, which featured a 360° turrent and a 75 mm gun, but at the time of the war, the Germans had already created the Panther mediums and the Tiger I & II respectively, which heavily outclassed the M4A3 & M4A4 that were created during the time. To address this situation, the M4A3E2 & the M4A4 Firefly were created to help in the break-in sector and the tank elimination zone. The U.S. also took the time to address their Air force, in which their planes had a heavy firepower & armor approach, which did not work as well on the older planes, leading to the development of the P51 Mustang & others like the Corsair, which also added better engines to adjust to the speed of the Mitsubishi Zero, which relied on speed and accuracy to defeat the Allied planes.
3) Political Policy and Global Events: This closely follows the Mixed Population, in which the U.S. government under Roosevelt knew that choosing was not the best decision without the right circumstances. As of World War I, the US chose to follow the policy of Isolation, and only took notice to help the Allies following the defeat of France. The U.S. had their circumstances following the Bombing of Pearl Harbor and the declaration of war by Germany.
The policy of Isolation was the correct move on the part of the Roosevelt Cabinet. Failing to get adequate reasons would lead to inner turmoil, and the destruction of the extremely frail economy that had just arisen from the Great Depression, as well as disintegration of public relations in a time when the people needed to come together.
