I remember watching a few years ago a Serb socio-political talkshow called
Ćirilica (Cyrillic) hosted by Milomir Marić. When Marić asked if Yugoslavia could have survived if she joined the EC one guest (whose name I forgot) said that the better question would be if the EC could have survived us. The Balkan peninsula has this habit of causing long and agonizing deaths of empires that tried to rule there.
IIRC one of the main, if not the main reason, was because the country was at this point an utter mess. Older people from the region might fondly remember
this newsreel. Remember: By 1990 the communist party of Yugoslavia, and thus the last uniting element, was no more, the six republics were acting like states within states, the local communist parties (with the exception of Serbia and Montenegro) were kicked out by local nationalist and thus alienating the republics even more, in Croatia and Serbia respectively the Krajina Serbs and Kosovo Albanians created para-states and thus states within states within states, the only thing of Yugoslavia that was left at this point was the federal army, the jackals and hyenas were already waiting for Yugoslavia to die so they could feast on her carcass while sabotaging Ante Marković's attempt to save what could be saved and the list goes on. Milošević and his alpha bitch of the JUL is a tale of itself...
If the EC turned Yugoslavia and her republics into a client state of Bruxelles then
maybe there was a chance. However Falecius made a good point that Bruxelles had much less jurisdiction back then. An early admission of Yugoslavia could also backfire horribly. Imagine the only thing the EC could do (besides of pouring billions into a black hole) is watching helplessly while Yugoslavia descends into a civil war like IOTL. This could damage if not outright kill not only the credibility and prestige of the EC but also the very idea of a "united Europe".