Turkey:- 97% of Turkey is in Asia, the eu does not need to share its borders with Syria, Iran and Iraq. Agreeing to one non European country would and could open the door to countries from Cape Verde to Kazakhstan.
Macedonia and their dispute with Greece could and probably would see Greece blocking Macedonian membership from the EU and Nato, again creating hostile relations between the two.
Serbia has far longer-standing, and more permanently damaging problems, that are slowing and halting its progress in many directions, firmly rooted over decades in social structure
But hey stranger things have happened, I mean Saudi Arabia was given a key role of human rights by the UN!