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—may be paywalled.)
Hitler (1945-04-29, as quoted by Traudl Junge in Until the Final Hour, 2005, Phoenix, p. 175):
Germany is lost. It was probably never mature and strong enough for the task I intended it to perform.
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.