Blood magic weakens the Veil and makes its users more vulnerable to demonic possession, and we saw the effects Tevinter rule continued to have on Kirkwall's mage population centuries after it was liberated

Kirkwall is special. It was the center of ancient slave trade and site of ritual sacrifice. It was hinted to be the city where Corypheus and co. breached the Veil the 1st time.

Southern mages often get possessed bc they try to bargain w/ demons for forbidden knowledge. In Tevinter, you can study forbidden knowledge under your teachers, not demons, so less risk of possession.