in 1994, Bulgaria and Romania were in the semis and quarter finals of the world cup and yugoslavia (serbia) would popably have been there. afterwards they were regular contenders at euros and WC until 2000 and from. then, evetything went downhill with only some participations Bulgaria 2004, Romania 2008, Serbia at WC 2010 and 2018 but not reachong any knock out phase. on the other hands, northern and central east euro nations (poland, czech, hungary, slovakia) face some kind of revival. in the last decade. what are the reasons behind that developments?
The economies of Poland and Hungary have bloomed while Romania, Bulgaria, and the Balkans have been plagued by corruption.
Financial resources (or lack thereof) play a part, but realistically Romania and Bulgaria were just having a Golden Generation moment in the 90s. Prior to that they had only made a few competitive group stages.
Croatia's success at reaching the 2018 World Cup final maybe suggests that this trend isn't entirely accurate.