Why didn't the soviet union just keep their part in the occupied austria? They could do something like west germany which could help them to expand the iron curtain. So why would they leave austria peacefully?
This is a deeply complex topic, but one well addressed by /u/kieslowskifan in their response to "Why did Austria not wind up behind the Iron Curtain?" in this subreddit. Several other responses they made in that thread give excellent coverage that should help answer your question.
To quote u/jeanpauljh when I asked a similar question a few months ago.