Hitler (1945-03-19, as quoted by Albert Speer in *Inside the Third Reich*, 1970, Macmillan, p. 440):

> For the nation has proved to be the weaker, and the future belongs solely to the stronger eastern nation. ([Google Books][1]—may be paywalled.)

In Hitler's crude and mystical Darwinist survival-of-the-fittest world-view, Nature decrees that stronger races/peoples defeat weaker ones.

The Germans lost simply because they were weak while the Russians/Slavs were strong. That was the fundamental reason and all else were mere unimportant details.

  [1]: https://books.google.com/books?id=lt4ZPz8n53cC&q=%22the%20nation%20has%20proved%20to%20be%20weak,%20and%20the%20future%20belongs%20entirely%20to%20the%20strong%20people%20of%20the%20East.%22