Why did the German retreat from South Eastern Europe happen so quickly?

After the Soviet Jassy–Kishinev Offensive during the late summer of 1944 the German army quickly retreated upwards towards Nazi Germany through Hungry, Czechoslovakia and Austria. This meant that almost all of South Eastern Europe (Greece, Yugoslavia, Bulgaria) was liberated without a fight. Why didn't the German army carry on fighting like it did elsewhere at that time? Could you explain in detail.



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retreats