Mostly a numbers game but some pretty bad decisions were also made especially later in the war. The US was the largest industrial power by a wide margin and practically immune to attack on the mainland. Germany could never match the sheer output and in a total war the larger economy is going to win eventually.

The biggest strategic mistakes were going to war with the US when they didn't necessarily have to, choosing to fight a 2 front war and continuing to fight Britian when they had no real chance of winning quickly. Really you would need to change all 3 of these for Germany to have any real chance of winning.