Mostly because Adolf Hitler was an idiot. His two biggest mistakes were (a) invading the Soviet Union and (b) declaring war on the United States. If he had quit after the Fall of France, it is not likely that Britain would have been able to liberate Europe, or that the United States would have gotten involved. As a result of the attack on Pearl Harbor, there was widespread support in the United States for war on Japan, but not for war against Germany. Notice that FDR did not immediately ask for war against Germany. A few days later, though, Hitler rashly and foolishly declared war on the United States, which is all FDR needed. We were off and running, and it was the beginning of the end for Germany.