There were several reasons:

Stalin, as early as 1943, had been an advocate of an independent Austria. In the Moscow Declaration of 1943, it was basically the Soviets that put the paragraph about Austria being the first victim of National Socialism should they contribute to their own liberation in there. It was seen as useful way of making Germany weaker and also establishing a post war order in which Austria would not part of Germany.

Another thing was that Stalin died in 1953 and the Soviets under the collective leadership after Stalin's death were looking for ways to relax the relations to the West a bit and a neutral Austria fit right into that political concept.

Thirdly, the strong West alignment of Austria that was established wasn't exactly foreseeable. Austria was to be neutral like Switzerland. As a fellow Austrian I can see how ridiculous this may appear from what we know now today but back then, the strong Western alignment was as clear.

Fourhtly, even if the strong Western alignment had been foreseeable, the idea of a neutral Austria befitted Soviet strategy in Europe since it was highly unlikely that Nato would violate a countries neutrality in an attempt to attack the Soviets and without being part of Nato violating Austrian neutrality was an easy plan for them themselves.

Sources:

• Andreas Hilger,Mike Schmeitzner,Clemens Vollnhals: Sowjetisierung oder Neutralität?: Optionen sowjetischer Besatzungspolitik in Österreich

• Oliver Rathkolb's document collection on Soviet policy in Austria.