Ok, since you’re getting into medieval territory, first off let’s say that the Turks weren’t really all that numerous and powerful during their initial conquests on the Balkans. On the contrary – scholars on this period have said that “the Turks conquered the Balkan Christian nations with the help of the same Balkan Christian nations”. In other words, the Turks were good in following the old maxim of “divide and rule”. Especially during their initial conquests of the Balkans you can see how much effort they put into vassalizing the various rulers and lords on the Balkans, thus using their armies for their own conquests. Hence f.e. you have Krali Marko/Marko Kralevic, the greatest hero of the South Slavic heroic epos (at least for Bulgarians and Serbs), but in reality he was an Ottoman vassal and died in Wallachia, fighting for the Ottomans. The same way you have Serbian cavalry in the Turkish armies of both Nikopol and Ankara. So that’s the main strength of the early Ottomans and their conquest – making perfect use of the decentralized, disunited Balkans.

In that respect, there’s a good quote from one Bulgarian chronicle from those times:
“And when Kantakouzenos saw that he can’t feed his soldiers on the ships, because bread and pork were not enough, and the treasury itself was being emptied every day and there were no more dukats and silver, he thought and sent messengers in Tarnovo to the Bulgarian Tsar Alexander, so he could help him with the supplies for the marines and they could both protect the straits together. But when the Bulgarians heard this, they laughed and swore at the Greeks, not only by insulting them, but also by swearing them on mother and wife, and then sent them away empty-handed. When he saw this, Kantakouzenos was struck with sorrow and sent messengers to the Serbian lords – Urosha, despot Uglesha and king Vukashin – to support the marines. But they also laughed and cursed at the Greeks, not only insulting them, but also by swearing them on mother and wife, and sent them away empty-handed. When Kantakouzenos heard this, he was struck again with grief and wondered what to do. Then Kantakouzenos sent to the Bulgarian tsars and the Serbian lords a message, telling them: “You didn’t help us now, but you will be sorry later!” But they payed no attention to these words and replied: “When the Turks come against us, then we’ll defend ourselves from them”. Then Kantakouzenos made an agreement with Amorat, they made an oath and gave each other letters, which are still being kept today, so the Turks wouldn’t damage the Greeks for all times, neither in Rhomania, nor in Macedonia. The Turks promised to keep these terms and then Kantakouzenos allowed the Turks to cross through Gallipoli.”

In other words, on the Balkans everyone in that time – be it Byzantines (let’s not forget that it was exactly they who brought the Turks to the Balkans in the first place), Bulgarians, Serbs or others – everyone looked only after themselves. And not just on a national level – the 14th century Balkans were a very decentralized place (thanks to the new feudalism), where every lord and bandit would try and carve up a domain of his own. We can clearly see this in Serbia after Dushan’s death and its various “successor states”, we can also see it in Bulgaria where brother separated from brother (as well as the despotate in Dobrudzha), not to mention the Latin states in Greece, which were divided by nature. Heck, going back to Marko, we have his uncle – Momchil – who was literally a bandit and mercenary, who also carved a short-lived domain of his own. That’s the situation the Turks found on the Balkans and it’s not surprising that they managed to make such a good use of it.