The quick answer?
Geography. The U.S. is bordered by two large oceans.
Raw materials: Where Germany and the Axis powers had to invade to obtain the materiel to either make, or run their war machine (reads gas, oil, & steel), America had stores of natural resources....that and the fact that our vast oceanic borders kept the raids on our war machine/factories at bay----not so the axis. We took the war to them.
Mass production: Borrowing models based on the Henry Ford production line of interchangeable parts....we simply overpowered the axis with our ability to mass produce guns, bullets, and beans. We had a liberty ship going down the skid way every week. While Germany made magnificent weapons, particularly their tanks(armor) and Navy(uboats).....they took much longer to build based on their complexity, and then, there was the matter of fuel consumption to run them, and their ability to repair them...complex equipment need complex parts to make them run again, and they simply couldn't pull one off the shelf. By contrast, the U.S. out-produced the axis based on that Ford model.
Yamoto said it best after the attack on Pearl Harbor; "I fear we have awoken a sleeping giant......"