I'm not sure how this is an EBS question. It appears to be simply a question about why the question of Kosovo causes problems for the ascension of Serbia into the EU.

Most European Union members recognize Kosovo as an independent state, with only a handful withholding recognition, often for reasons that don't have anything to do specifically with how they feel about the conflict between Serbia and Kosovo (e.g. Spain is more worried about the impact recognition would have on its own secessionist groups than it is about the particulars of the Serbia-Kosovo situation).

If Serbia is admitted to the union, it would wield significantly more influence within the European community than it does right now. EU decisions must be agreed upon unanimously by the union members, and if Serbia were admitted to the union, it could use that unanimous requirement to exact concessions regarding Kosovan sovereignty from the countries who have decided to recognize Kosovo as an independent state.

So the countries that have acknowledge Kosovo as an independent state want Serbia to make concessions on Kosovo before joining the EU to reduce the possibility of that issue resulting in conflict down the line.