u/kieslowskifan gave an excellent answer to a very similar question on this topic.

A one-paragraph summary might be thus: Poorly thought out planning made occupation costs higher than expected, and there were grave doubts about Austria's economic and political viability as an independent state. At the same time, neither the USSR nor the Western Allies wanted the cost of occupying (and therefore propping up) newly-independent Austria; but neither side wanted to allow the other side to possess it and use it for staging military assets. An agreement was reached in the mid-1950s that allowed Austria to be independent but with enforced neutrality.