Usually minorities that want to separate are richer than the rest of the country. Think of the Basques and the Catalans in Spain and the Slovenes and Croats in Yugoslavia. They have a good reason to believe that when they no longer have to "subsidize" the rest of country they will be better off.
Sri Lanka Tamils and Kosovo Albanians are exceptions to that rule. Interestingly, when you compare them there are many similarities:
- in both cases there is a neighboring state/country (Tamil Nadu and Albania) where they dominate. Tamil Nadu is part of India but as a state it has considerable autonomy.
- in both cases they have in the past been either independent or part of that neighboring country.
- in both cases they collaborated with a foreign ruler (the British resp the Ottomans) against the majority. In that period they were the most prosperous group.
- in both cases that led to retributions after the departure of that foreign ruler.
- in both cases the national majority attempted to "majoritize" the area after they achieved power. In Sri Lanka a few years after independence in 1948 laws were adopted that made Sinhala the only official language. In Kosovo there was a not very effective policy between the World Wars that encouraged Serbs to move to Kosovo and Albanians to emigrate to Turkey.
- in both cases their recent nationalism is strongly supported both from the neighboring country and from the emigrant diaspora.
- in both cases this results in a kind of rejectionist nationalism where people refuse to learn the majority language and also in other ways isolate themselves from the mainstream economy. The resulting poverty is blamed on the majority and so one gets a self-reinforcing pattern.
- through time the situation gradually worsens with increasing mutual discrimination and violence.