The Eurovision Song Contest has been running in Europe since the 1950s, however none of the Eastern Bloc/Warsaw Pact countries took part until the fall of the USSR/end of the cold war. Why not?
The Eurovision Song Contest is organized by the European Broadcasting Union (EBU), an alliance of public service media entities. Only members can participate. Now, the EBU was a firm Western European group, led by the BBC during its formative years.
The eastern bloc did not have public service media entities, and the USSR was somehow suspicious about a BBC led organization. They never joined and remained in the International Radio and Television Organisation (OIRT), which they transfomed into an eastern bloc equivalent of the EBU, forming Intervision as counterpart to Eurovision. They even launched a competitor to the Eurovision Song Contest: the Intervision Song Contest.
Steve Rosenberg: The Cold War rival to Eurovision. BBC News, Moscow. May 2012.
Radio Free Europe: Intervision: Eurovision's Poor, Defunct, Ex-Soviet Rival.
Heimann, Thomas 2006: Television im Zeitalter des Kalten Krieges. Zum Programmaustausch des DDR-Fernsehens in den sechziger Jahren. In: Lindenberger, Thomas (Hrsg.) 2006: Massenmedien im Kalten Krieg: Akteure, Bilder, Resonanzen. Köln, S. 235-262.