My theory is it's because they're mostly depopulated being ground zero for Earth's nuclear war:
Khan was the last of tyrants and ruler of the largest land mass on the world from Asia through the Middle East. Though not specifically stated, nuclear war was the outcome. Being the last of the tyrants to be overthrown, his kingdom (Asia/Middle East) would be most likely be ground zero for most of the nuclear exchange. Here's a map of Khan's potential kingdom for reference (note that India and China would be affected most):
(Also note, there's a little over 40 countries in black labels, so, I have the controversial opinion that this region was initial source of the Eugenics War, and hence, explains this region's low representation in-universe for Star Trek.)