There is a small issue in the premise of the question.

Many Serbs, Greeks and Bulgarians did convert to Islam during Ottoman rule. It's just that we - in English especially - don't have a name that's in common use.

Greeks: The Greek-speaking Muslim population was largely deported to Turkey in the 1920s (as the Turkish-speaking Greek Othodox population was sent the other way). These Greeks would sometimes use the Arabic script to write Greek as a nod to their religion, just as the Greek alphabet was used by Christians to write Turkish.

Bulgarians: Pomaks might not be commonly-known in English, but it refers to Slavic converts to Islam in Bulgaria. As with the Greeks, many of these ended up migrating to the Ottoman Empire and later Turkey, where they are often known as Bulgar (Bulgarians) or muhacir (migrants). Bulgarians in Turkey are often known for being distinctive physically (blue-eyed, blond, etc) from the general population. There remains a sizeable Muslim community in Bulgaria today but they are known simply as "Turks" and speak Turkish.

Serbs: Many simply consider Bosniaks and Serbs the same group, except the former has converted to Islam. It is fair to ask why that conversion was concentrated on the frontier rather in what is today Serbia.

Overall, a characteristic of Bulgarian/Greek converts to Islam from Albanian and Bosniak converts is that they gradually adopted a Turkish identity while the Albanians and Bosniaks have retained a distinct one.

The answer is to some extent geographical. Bosnia lies between Croatia and Serbia, the former was heavily catholicized by the time the Ottomans showed up, and the same is true for Serbia, the difference in that Orthodox christianity dominated Serbia. Meanwhile, Bosnia was divided between the two branches of christendom, add to the fact that the Bosnian church's power was not particularly wide-reaching and religion in Bosnia was not as centralized, and you get a simpler task in converting local slavs to Islam. To add to the geographical context, the Bosnian terrain is very mountaneous and its settlements were not easily reachable for Chistian pilgrims from Rome or Constantinople, thus many Bosnians were quite unaffected by religion in their everyday life. For instance, the Bosnian church was considered heretic at one point by the Vatican. Sorry for my hastily written contribution.

I have no knowledge on the reasons for the high conversion among albanians.