Why did Yazid ibn Mu'awiya appoint Ubaydullah ibn Ziyad to the governorship despite his wickedness?

Yazid was not a just Imam; rather, he was one of the imams of injustice, oppression, and immorality. No mercy is invoked upon him due to his committing grave acts, the most severe of which was his killing of Hussein ibn Ali (may Allah be pleased with them both). Al-Dhahabi said that he was a harsh, partisan individual who consumed intoxicants. He began his rule with the killing of Hussein and concluded it with the Battle of al-Harra, causing people to detest him. His good deeds are overshadowed by his evil deeds, and we leave his matter to Allah. We neither curse him nor love him, and refraining from cursing him is more preferable.