Factors that helped the Soviet Union turn the tide against Germany in WWII:
German troops had to cover long distances.
Colder climates created problems that the German military could not overcome.
The blitzkrieg relied on tanks that were most effective on flatter terrain.
An invasion of the Soviet Union in 1941 posed problems that the Germans had not confronted in the earlier campaigns.
Geographically, western European Russia was dominated by the Pripet Marshes which ensured German operations in the south would be isolated until well into the Ukraine. Equally, the sheer size of the Soviet Union was a powerful influence upon the effective conduct of operations.
German armored units had suffered wear and tear in the short French campaign. It was only 322km (220 miles) from the Ardennes to the Atlantic, but it was 1609 km (1000 miles) from Warsaw to Moscow and 1931 km (1200 miles) from Leningrad to Rostov.
Stalingrad was 3219 km (2000 miles) east of Berlin. Once Hitler declared his intention to move east on July 30, 1940, planning was dominated by the desire to win one campaign. Yet, the Russian climate limited the mobile warfare to between May and November.
To learn more, see the following sources below:
Stalingrad: The Infernal Cauldron, 1942-1943 By Stephen Walsh
CISD.org - World War II 1939–1945
History.com - Battle of Stalingrad