Why Germany couldn't held a war of resistance using militias against the Allies, even though the German deep forests seem a good place to organize this kind of warfare? What did the Allies made different before the invasion, so they didn't have to face a resistance opposition, as Germany did when it invaded Poland and France?
Short answer is: people had enough of any sort of military action. Also, they saw how manipulated and disillusioned they were by the Nazis. There is no successful guerilla force without local help or a cause that could rally people up. This one could not.
Shortage of men (many killed) Realizing the lies and manipulations of the Nazis fed up with war no supplies absolutely overwhelming enemy forces