Tl:Dr Because, despite initial doctrinal advantages, Germany lacked the industrial capacity and manpower to fight a long.

Short story long, when Germany started the war Hitler was reliant on a series of rapid victory's, won through Blitzkrieg, which was a tactic born out of this necessity. Initially it was thought that once France collapsed Great Britain would sue for peace, the USA would remain uninvolved, and then Germany would be free to deal with Russia, again, ideally through a rapid victory. The reason for fighting the war like this was Germany's comparative size and industrial capacity (not to mention that the government was completely broke, and reliant on foreign credit from the Soviet Union). At the beginning of the war Britain alone was producing as much or more steel and aircraft as Germany was, let alone the USA and Soviet Union (Len Deighton's, blood, tears and folly). And where the other powers were able to ramp up production to build numbers, Germany struggled throughout the war to replace it's losses. As an example the Luftwaffe which lost the battle of Britain was the same size when operation Barbarossa began, so an airforce that couldn't dominate the south east of England was always going to struggle to maintain control over the vast eastern front.

Basically the picture I want to paint here is a country with a large, well trained armed forces going g into the war, but without the ability to replace it's losses. So once the allies were able to stop Germany's initial advances and update their own doctrine for the relatively new type of war, they were always going to win as they could afford to suffer much, much greater losses of men and equipment.