I've never had an answer for this question, even after visiting the Austrian military museum in Vienna and interviewing immigrants from Austria. When drawing the "iron curtain" line in Europe, the border turns sharply eastward to include Austria and then returns west for Yugoslavia. It seems that Austria was almost forgotten by Stalin! Or maybe he was tired after annexing East Germany.
Well, a couple of reasons. First off, the Americans got to at least big parts of Austria in time to ensure that the Soviets lived up to the wartime deal.
But the deal itself was the main thing. Like Germany, Austria was cut into four sections (Soviet, American, British and French). Vienna was chopped up like Berlin. So the Western Allies could stay as long as it took to set up a neutral state. If the Soviets insisted on a satellite, the Western Allies would stay forever.
The Soviets decided to go for a true neutral. This arrangement was enshrined in the State Treaty of 1955. The Austrians were not allowed to join ally alliance. The army was prohibited from having some equipment (Like oddly,rockets and missiles; the Austrians specialized in anti-tank guns for years.) No outside forces were allowed in the country.