It was ok for Kosovo, so why not Crimea
The legality of the declaration and whether it was an act of the Assembly has been disputed. Serbia sought international validation and support for its stance that the declaration was illegal, and in October 2008 requested an advisory opinion from the International Court of Justice..
The Court determined that the declaration did not violate international law.
So, why not Crimea?
United Nations Security Council Resolution 1244
also see Mayotte an island off the coast of southern Africa, which is the newest region of the European Union and has EU protection in international law through the provisions of the Lisbon Treaty as of 1 January 2014.