The same reason they aren't part of serbia. The region overall is hard to determine when it comes to ethnic and historical issues with it you see
Ethnicly it has majority Albanians but there are regions where serbs are dominant, there's also mix of bosniaks, croatians from old Yugoslavian times. You see when it comes to ethnic situation but historically it was for more time part of Ottoman Empire than Serbian, Albanian. Basically Kosovo probably is willing to create a new history for their little nation, rathered to separate from Albania and Serbia because their fighting would be eternal over a small piece of land while region wouldn't benefit at all because all the infightings make the region degress. So there we go, they got with idea why not make land where serbs and albanians can live in and write equal history there. Obviously some people recognise it, some do not. I do and its my personal belief not only because UN majority recognises it but also that from the people perspective they have enough of wars and probably just separated from both states but in reality they speak the same tongues. It's not like they can diplomaticly befriend both states and keep in check everything together there, funnily enough Switzerland in past did the same by distancing themselves from HRE (which was proclaimed as basically German entity, decentralized union of german states with duchies principalities free cities etc no need to explain it was later announced that Switzerland would be free from HRE just like Italian states did before. So you see these things happen because of reasons they are either solely economical or the most approved among the population they bring unity and sometimes, solve the crisis.
xD[ sry for essay]