King Saul proved to be an unfaithful king. He was prideful, disobedient to the Lord, and self-reliant. He consulted witches and mediums instead of praying to the Lord. He was irreverent, and even tried to function disobediently as a priest of the Lord. Because of this he was rejected by God.
1 Samuel 15:23
“For rebellion is like the sin of divination, and arrogance like the evil of idolatry. Because you have rejected the word of the Lord, he has rejected you as king.”