The ultra-conservative Serb nationalists were in charge of the country when Yugoslavia broke up, and they promptly started wars of aggression with those former Yugoslav republics who did not want to be called Yugoslavia anymore.
Then they engaged in ethnic cleansing and the worst incident of European genocide since the days of World War II, and the international community took note. NATO intervened and bombed the ever-loving stuffing out of Serbia. As a result of the prolonged civil war and the subsequent smackdown by NATO, the Serbian economy is still in shambles.
After the smoke had settled, Serbs elected themselves a new moderate government, and their old head of state died a well-deserved death while on trial for war crimes.
Well, Serbia recently had elections once more, and the majority of Serbs decided to give the old ultra0conservative nationalist party another try, because apparently they hadn't totally destroyed the country when they were at the helm, and maybe they deserve another chance?
Wave a flag and collect votes. It works the same in any country, really.
Civilization is a very thin veneer, folks. Nationalist and religious grudges run deep, and even a modern industrialized nation whose people sold pizzas and boat rides to German tourists for the last 40 years can and will revert to savagery if the right set of circumstances are met. All that's required is perceived social status and moral absolution from his fellow citizens, and your friendly neighborhood grocer will happily grab an AK-47 and start kicking out gold teeth.