Because the rule in the "balance of power" game is not based on "Will" but on "Can" and it dictates that if Croatia is more powerful than Serbia it will launch an attack. Its reality, nothing more. That is the main reason for ongoing arms race in the Balkans and those things, well, never end well. Oh, but they do. Unfortunately discussing the reasons for it, is not the subject of this thread but I will try to explain in the autopsy thread.
Granted relations with Croatia aren't good at the moment, the thing is relations between both nations have seen their ups and downs since the breakup of Yugoslavia and today is no different. Some local town burning effigies of the other sides politicians in public square may be in poor taste but it is not a sign of imminent war. As I said before Croatia would be kicked out of the EU if it ever attempted to attack Serbia. The chances of such a war breaking out in this generation is as near to zero as you can get. Climate change is probably a greater threat to Serbia going forward than Croatia will ever be.