Why did Hitler fail to invade Russia in 1941?

The short answer is that Hitler was unprepared to fight a long war in Russia during the mud season (Rasputitsa) and against a Russian people so zealous in their patriotism and belief in the red state. Hitler started his invasion (Operation Barbarossa) in June, between Russia's two annual mud seasons, but believed in a quick victory and didn't supply his men with adequate coats, shoes or vehicles. Light tanks or treaded vehicles are perfect for traversing muddy areas but Hitler sent heavy tanks which sank and jeeps that became stuck. The standard Nazi 'Blitzkrieg' that had worked so well in Europe, failed in Russia and the German army became bogged down as mud season and winter took hold. Hitler consistently changed his tactics and lost large numbers of men with no reward. This was compounded by a Soviet army that was willing to make last stand after last stand and were much more prepared to fight on their home soil. While the Russian forces actually lost more men than the Germans, they had more men to lose and the value on one of their soldiers was much lower in this campaign. Germany was forced back in August 1941, and then finally expelled in 1943, spelling the end of German attacks on Russia in WW2.