Same reason no one interfered with US invasion of Dominican Republic. It was our sphere of influence. Just like Eastern Europe was USSR's. No one was willing to risk global war. During the Cold War it was the countries more distant (Vietnam, Angola, etc) that were really up for grabs and fought over. The Truman Doctrine basically stated this. We wouldn't try to rollback Communism in Eastern Europe, but we would fight to prevent it in the 3rd World.