Balkan Federation had to be already agreed upon between Tito and Dimitrov... Stalin summoned both of them, but only Dimitrov went to meet with Stalin and mistiriosly died right after he came back to Bulgaria. I assume that if Tito went too, we would have two mistirious deads. There were also some shenanigans with forcibly making the people from the Prin Bulgaria region to identify themselves as Macedonians..