I only have details about the siege and conquest of Constantinople from the documentary on Netflix. Therefore, my knowledge could be relatively sketchy.
During the siege of Constantinople, the Byzantines desperately waited for papal troops. The only thing that was heard about aid from the Balkans was that the Branković family thought about sending aid, but that never happened. But why didn't the Serbs and Bulgarians send help immediately? Because they were relatively close to Constantinople and they knew full well that the Ottomans would be the next to conquer their lands if Constantinople falls. In the end, that's exactly what happened.
3.2 Economic benefits from the conquest of Constantinople. After the conquest of Constantinople, the Ottoman Empire grew in popularity. Thanks to the good population policy, the population increased. In Istanbul it rose to 90,000 while Mehmed II was still alive.
The Bulgaria wade old enemies of Byzantium, fought many wars with them and plundered their cities. At times they controlled the entire European part of the empire.
Ok but to what extent was that an advantage for Serbs and Bulgarians?
In 1396 the last stately Bulgarian structure - Kingdom of Vidin - fell under Ottoman rule. Thus at the time of the fall of Constantinople - 1453 - there had been no Bulgarian state power for almost 60 years that could provide or refuse aid. All living heirs to the throne and their successors are in exile.
Serbia was skillfully manipulated by the Ottomans at the time. The Serbian princess Mara Brankovic was married to the then Sultan Murad II in 1431. After his death in 1451 she was sent to her parents' court by the new Sultan Mehmed II as a kind of "influencer". Most likely she exerted decisive influence on her parents during the siege and fall of Constantinople in 1453, so that Serbia does not take sides against Mehmed II.
