Bosnia, Albania, and Kosovo have large populations of Muslims. At the same time Serbia was under ottoman rule longer than Bosnia and in some ways influenced more by ottoman culture than Bosnians. Also how peaceful were this conversions?
Peace be upon you. More can be written, but you might like to start with the "The Balkans" section in the FAQ, with answers by /u/UrbisPreturbis and /u/vonadler .
This is not to discourage discussion. More questions, data, and debate are welcome.
The Serbs and the Ottomans had some sort of alliance that goes back to the crusades. In the last crusade battle, the Serbs were fighting with the Ottomans. Look up the Battle of Nicopolis and Stefan Lazarevic.
On the other hand the main battle line was eith the Croats, and the Posavina Croats in Bosnia were slaughted by the Ottomans so thaf in case of a battle they cant fall in the back of Turkish solfiers. And the Turks killed and punished them for all sorts of reasons, and they made it obligatory in Bosnia that you had to be Muslim to do anything more important (applied to Bosnian Serbs as well).
Albania and Kosovo is a different group of ppl (not Slavic), I don't know anything about them.