Why did the rest of the Eastern Bloc not invade Yugoslavia? Yugoslavia
It was a real possibility, especially after Marshal Tito blocked Stalin's supplies for the Greek Red Army in 1948, collapsing the Soviet sponsored revolution in Greece after WWII. But In 1953, Tito reached a mutual defense treaty with Turkey, which ultimately meant Yugoslavia was backed by NATO, while possessing a pretty significant armed forces of his own. Yugoslavia was after all one of only two countries in Europe, the Soviet Union being the other, which was able to push the Axis occupation out largely by themselves. (Soviet's and Bulgarian troops provided significant help to liberate their capital).
Tito was a master of playing the West and East against each other. His country requested and received aid in 1951 from the United States, during the the Marshal plan. At times during the cold war Yugoslavia recieved aid from both the West and Soviets.
Why didn't the Soviet Union invade Yugoslavia.
First, Yugoslavia wasn't a pushover. They had fought a long and bloody civil war which overlapped with WWII. They had given Hitler and Mussolini all they could handle and then some during WWII and Stalin wouldn't lightly involve himself in a war which he might not win.
Second, Yugoslavia, while not a member, was aligned with NATO. Yugoslavia formed the Balkin Pact with Greece and Turkey. Which pledged each to come to the other's defense in the case of Soviet invasion. Given Greece and Turkey were NATO countries this effectively placed Yugoslavia under NATO's security umbrella without the provocative move of actually formally admitting Communist Yugoslavia to the alliance.
The combination of both were enough of a deterrent to keep Stalin from invading. Although the Soviet Yugoslavian relationship definitely saw its strains.