The long and short of it is the logistics situation. The logistic capabilities of the Wehrmacht were severely lacking (especially compared to the western Allies), and Russia, at the time, was a particularly difficult area to run a modern war in. German propaganda reels of the time repeatedly complain about the terrible roads in Russia (in the way of "look how these Untermenschen build roads!"), it is a very big country, its rail network runs on a different gauge than the rest of Europe and twice a year it drowns in mud. And that is before the Red Army even factors into things.

None of this was news to the Wehrmacht, no matter what they would claim after the war. It became apparent during planning Barbarossa that logistics would be difficult at best. Therefore, the Wehrmacht decided that it had to fight a short campaign, destroy the Soviet armies in the field and end the war before the winter starts. Of course, to do that you need to engage the enemy in intense fighting and drive your forces as hard as possible to reach your objectives in as short a time as possible. So the priority of supplies obviously lies with ammunition, fuel, spare parts and replacements. Winter clothing is a distinctly lower priority, but that's not going to be a problem because you will win the war before winter starts, right?

On top of that came the impression the Germans had of the combat capabilities of the Red Army. The Soviets had not exactly covered themselves in glory during the Winter War against Finland, and the Nazis, being Nazis, naturally assumed that the German soldier was superior to the Soviet one in all aspects. So a quick victory was considered very much possible, which meshed well with what the German high command knew needed to happen for a successful campaign.

It should be noted that the western Allies made similar considerations during the summer and Fall of 1944, leading to a distinct shortage of winter clothing during the following winter. It should also be noted that the Red Army had similar logistical problems during the summer of 1941, which severely reduced their ability to resist the German attack. Logistics are hard.

Sources/Further reading:

Martin Van Creveld, Supplying War

Robert Forczyk, Schwerpunkt