Hey Tristan,

I briefly discussed this in an answer to a related question a few weeks ago. Allow me to go into a little more detail here.

There were a number of different plans on the table for the Third Reich when the war ended in 1945. One of the most (in)famous is the Morgenthau Plan, named after the US Secretary of the Treasury Henry Morgenthau. In his book Germany Is Our Problem (october '45) he proposed that the German Reich should be divided in an international zone stretching from the Dutch/Belgian border to the Weser river, a North German state east of the Weser river and a South German state encompassing roughly Bavaria and Baden-Württemberg. North and South Germany were to be de-industrialized thoroughly to prevent them from ever becoming powerful, even if that meant that many German people would possibly die of hunger and economic malaise. The international zone in the west meanwhile, would only be partly de-industrialized and would by ruled an United Nations backed international authority.

Even though the divison proposed in the Morgenthau plan never became reality, its contrast to the Allied plans for Austria is stunning. While Nazi Germany was (logically) judged by the Allies as the perpetrator of the Second World War, Austria was (somewhat oddly) judged more comparable to a country occupied by the Nazis. In the Moscow Declaration issued as a joint statement by Stalin, Churchill and Roosevelt in 1943, Austria was described as "the first free nation to fall a victim to Hitlerite aggression." While warning that "Austria (...) has a responsibility she cannot evade for participation in the war on the side of Hitlerite Germany", the declaration also stated that the leaders wished "to see reestablished a free and independent Austria, and thereby to open the way for the Austrian people (...) to find that political and economic security which is the only basis for lasting peace." ^(1)

While, militarily speaking, the occupations of Germany and Austria were similarily structured, this perceived difference in their responsibility for the war made their realities vastly different. Austria had an elected government by the end of 1945, while Germany was directly ruled by the Allied Military Council until 1949. There was no clear concensus between the Allies on what do with defeated Germany in the long term. On the other hand, the Allies had already agreed on the fate of Austria and they hoped to grant it independence as soon as possible. The fact that Austria had an elected government so soon after the war, already gave the country an amount of territorial integrity and political legitimacy that occupied Germany never had. In 1947 for example, two years before the division of Germany, France established the Saar Protectorate in a part of its occupation zone, a puppet state in the Saarland region on the Franco-German border. France doing the same in its occupation zone in Austria would've been unthinkable, since it would mean a crude violation of an Austrian government recognized by all other Allied powers.

Anyway, relations between the Soviet Union and the Western allies quickly soured after the war ended. Working towards a new unified Germany had become increasingly difficult, as suspicion abounded between the West and the USSR that the other would take over this newly independent German state. This discord was worsened by the opening stages of the Cold War, and resulted in the collapse of Allied military council overseeing Germany in 1948. Since there wasn't any form of overarching German state structure at that point, this basically meant that Germany was now divided along occupation lines. Compared to the deepening crisis in Germany, Austria was seen as relatively unimportant. While the talks on Austrian independence also collapsed, there was no attempt to further escalate the conflict on the ground. With the Austrian government being mostly pro-Western (or at least anti-Communist) by this time, there was no reason for the Western powers to do so anyway.

The Soviets on the other hand, never saw a communist eastern Austria as a viable idea in the first place. It would've been small and lighty populated: somewhat smaller in land surface than Belgium, but with only about 1,5 million people in population. Compare that to East Germany, which was larger than Belgium and the Netherlands combined and had about 17 to 18 million inhabitants when it became a state in 1949. Founding East Austria also would've been an odd breach of Austrian internal borders, since the Soviets occupied only a part of the state of Oberösterreich, with the Americans occupying the other two thirds. There were also worries in Soviet leadership that forcing an independent East Austria could lead the remaining West Austria to join with West Germany, something Stalin wanted to avoid at all costs. On top of that, the possibility that such a move could lead to a military stand-off between the Eastern and Western blocks was relatively high. There wasn't any reason why Stalin would want to risk all that for a small communist state that wasn't going to make all that much of a difference in the long run. When leaders of Austrian Communist Party requested Stalin to divide the country in 1948, he firmly rejected their request.^(2) After Stalin died in 1953, negotiations about ending the occupation in Austria picked up again, with the four occupying powers and the Austrian government signing a treaty granting Austria independence as a non-aligned state in 1955.

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1 - The Moscow Declaration, 1943, check page 12 / 827 for the part on Austria

2 - This interesting article by the Austrian professor Wolgang Mueller further explores the relation between the Austrian Communist Party and the USSR during the Cold War