For the same reason the number of Sudeten Germans decreased after the war.
It's just the Germans generally aren't stupid enough to try and deny what they wrought under Hitler, whereas the Serbs and their friends around here are stupid enough to try and deny what was wrought under Milosevic.
Never try to reason with a Serbophile. They'll try to convince you that Serbia was simply fighting Islam even though they attacked Christian Slovenia and ethnically cleansed much of Christian Croatia before they began attacking the Muslims in Bosnia (and many years before they went after the Muslims in Kosovo).