If Kosovo has the right to be independent why not Republika Srpska, the Bosnian entity?
First a bit of history. Republika Srpska was the creation of Dayton, the agreement that ended the Bosnian war in 1995. Its borders were computer generated. Bosnia's borders did change during the period of Ottoman rule but the area now designated as Republika Srpska was always included in those borders. Bosnia has pretty much held its current borders since 1878. The only real exception was the period of Royal dictatorship imposed in 1929.
By contrast, far from Kosovo always being Serbian, Kosovo fell under Serbian occupation only in 1912. Note – occupation. Legal sovereignty over Kosovo was granted not to Serbia but to the new Kingdom of Serbs, Croats and Slovenians after the First World War. Kosovo was established in its current borders since 1945 and in 1974 became a Republic in all but name.
But the real reason the two completely different is genocide. The genocide carried out on behalf of the state of Serbia in 1999 means that it is unthinkable that Kosovo will be returned, against its will, to Serbia. Given that the overwhelming majority of the Kosovan people are not willing accept a restoration of Serbian rule, independence is the only viable option.
In Bosnia it was the Muslims who were victims of an organized campaign of genocide. It was perpetuated by the same Serb nationalists who demanded Republika Srpska. Allowing the existence of Republika Srpska was an an act of appeasement. It is unthinkable that that campaign of genocide should be rewarded by allowing the Serb entity to become independent.