Many members of the German high command, and more importantly Hitler himself, believed that they would have captured most of western Russia (the most populated and economically important area), including Moscow and Leningrad by the end of the summer of 1941, and then the occupation could be figured out. This never happened, for a variety of reasons:

• The Germans already had a shortage of resources, and the start of the campaign was delayed over a month because of the need for additional troops in the Balkans.

• German high command believed that the Heer was invincible and would crush the Soviets with almost no effort. This was due to the success against France (which was seen as the leading European land military power leading up to WWII), and the fact that in the Winter War the Russians had massive trouble dealing with the tiny nation of Finland.

• Stalin and soviet command had learnt from the Winter War, and they were much more prepared than German intelligence knew. The red army was able to call up many more reserve troops than estimated, in a much quicker period of time.

• There was a diversion of troops southwards (to target kiev) before key Russian cities had been captured. It is likely that if Moscow had fallen (along with key soviet government institutions), the organizational capacity of the Russians would have been broken, which never happened.

Let's be absolutely clear on one point: there was no lack of winter uniforms. German planners were perfectly aware that the Russian winter can get quite cold and, regardless of dreams of a quick victory, they planned to be in Russia for the long-haul.

The problem of winter supplies, which should not detract from the wider operational issues that /u/jeremyjc has laid out, was far more prosaic. The strain of the war triggered a massive rail crisis in Germany during the winter of 1941-42. This affected the entire German economy, from coalfields in the Rhineland through to army depots in the east, but was particularly acute in the occupied Soviet territories. There the combination of factors – the sheer speed of the German advance, different Soviet rail gauges, the weather and, above all, the intense nature of the fighting – meant that by late 1941 the advancing German formations were right at the end of their logistical tether.

As a result there were severe difficulties in getting adequate amounts of anything to the front. Soldiers went days without rations, ammunition was in short supply and tanks were lying idle for lack of fuel. In this catastrophic supply situation greatcoats were not high on the priority list. Railcars containing such winter gear were simply shunted aside in favour of those carrying more essential supplies.

So it wasn’t that there wasn’t sufficient winter gear and it wasn’t that there were no plans to distribute this to the soldiers. There were plenty of winter uniforms sitting in warehouses in Poland. But the logistics crisis was severe enough that there were serious difficulties in getting this material to the front.

Sources

Stephen Fritz’s Ostkrieg: Hitler's War of Extermination in the East and David Glantz’s When Titans Clashed. See also Adam Tooze’s Wages of Destruction for a more general economic view of the winter rail crisis.

What /u/jeremyjc said.

But also note that military planners have a long history of neglecting this sort of long term planning. The US army was denying it needed winter clothing for troops in France just weeks before the start of the Battle of Bulge. Despite having been caught in exactly the same situation in Italy a year earlier.