I am learning a lot about the Yugoslav Wars of the 1990s, and a key point that Slobodan Milosevic brought up in the buildup to the large conflicts was that he wanted to unite Serbs into a single Serb state. He was concerned about all the states of Yugoslavia seceding because there were Serb pockets that would become minorities, and was worried they would become persecuted. And apart from small areas like Slovenia, all of the big Yugoslav states had notable minority Serb populations while other states did not have large pockets of their own ethnic enclaves to the degree Serbs did.
Why was it that Serbs had such a predominate diaspora among the Yugoslav states, rather than Croats, Bosniaks, Albanians, etc? And further, how did these enclaves survive so much strife over many generations that they could survive into the creation of Yugoslavia?
The distribution of different people in Yugoslavia and elsewhere precedes creation of common state, Yugoslavia. As a side-note, Yugoslavia means "country of South Slavs", meaning Slovenians, Croats, Serbs and Macedonians, later added Montenegrin too. Bulgarians were excluded.
Additionally, inner organization of Yugoslavia changed during time, so that inter-war period "banovina" does not correspond to post-WWII republic.
In Yugoslavia, all peoples that were not constituent were considered "ethnical minority" - Hungarians, Germans, Romanians, Rusyns, Slovaks, Bulgarians, Turks, Albanians, Italians etc.
The only republic that had significant Serbian minority was Croatia. It was result of moving Serbs to the border of Austro-Hungary to guard the border against Ottoman empire - hence name "Krajina", coming from "kraj" meaning end or limit.
In Bosnia Serbs were majority (42.9% in 1961) and it changed when "muslim" was recognized as ethnicity in 1971.
This is touchy subject, but "Bosniaks" don't have their own origin - they are people (Serbs and Croats) who accepted Islam during the Ottoman multi-century occupation of that area. Saying that, I respect fully their right to be separate nation and have separate religion. The case is different from that of Croats and Serbs who were two different South Slavic tribes who moved in that area in 7th century.
Kosovo was administrative unit inside Serbia, so while Albanians were majority there, there were minority in Serbia proper. Kosovo is additionally original area where Serbian state came to existence between 9th and 14th century. Demographic situation on Kosovo changed significantly during 500+ years of Ottoman rule there (1389-1912).
sources: census data, different books and text books on Yugoslav history.