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.
“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.” (1 Samuel 15:23)