Did Serbia actually exist after Yugoslavia's creation, though?
What did they base their claim about bulgarized Serbs on?
Before WW1, Kingdom of Serbia and the Kingdom of Montenegro were internationally recognized states. Croatia had autonomy within Austria-Hungary. Slovenia was divided into free states in the Austrian part of the country, while Bosnia and Herzegovina was under the control of the Austro-Hungarian Ministry of Finance and was a condominium.
By late 1918, Austria-Hungary ceased to exist. The Croatian Parliament abolished all relations with Vienna and proclaimed State of Slovenes, Croats and Serbs. That state lasted a month. The Italian army occupied Istria and Rijeka. Because of the fear that the Italians would occupy much of Croatia, especially Dalmatia, there was a unification of the State of Slovenes, Croats and Serbs and Kingdom of Serbia. Kingdom of Montenegro controversially united with Serbia, with a short-lived war between the supporters of two dynasties. Petrović-Njegoš was a Montenegrin dynasty, and Karadjordjevics were Serbian dynasty. Vojvodina declared a direct unification with Serbia.
After unificiation, King Alexander first divided the state into 33 areas. Croatian autonomy was abolished. In 1929 he divided country into 9 banovinas. In 1939, the banovina Croatia was proclaimed. It was just an administrative part of the state, without real autonomy.
Communists realized that such Yugoslavia was unsustainable. After WW2, new Yugoslavia was decorated by the Soviet model. The so-called AVNOJ borders were introduced. Socialist Yugoslavia was a federation of 6 republics. Therefore, Serbia was not an independent state, but it was part of the federation. Serbia again as an independent state was created in 2006, after Montenegro declared independence from the state union of Serbia and Montenegro.
Serbs considered the Macedonians as Serbs who, under Bulgarian propaganda, became a separate nation. Bulgaria and Serbia have been 8 times at war over Macedonia. Macedonia was an important part of medieval Serbia, and Macedonia was Serbian prize won after Balkan wars. Even today, the Serbian Orthodox Church doesn't recognize the Macedonian Orthodox Church. So the reasons are cultural, historical and geopolitical. Socialist Bulgaria recognized Macedonia as one of federative state of Yugoslavia, while Serbia had no other choice but to recognize independent Macedonia after breakup of Yugoslavia.