Answer:
King Arthur does not trust Mordred because Mordred had tried to steal the English throne away from him.
Step-by-step explanation:
Mordred was the illegitimate child of King Arthur and his half sister Morgause. Arthur did not know at that time that the lady was his half sister. Taking him in, King Arthur favored Mordred. But he tried to betray King Arthur's trust when he tried to marry Guinevere, King Arthur's wife and become king himself while Arthur was away to Gaul to declare war against Lancelot. Thus, King Arthur doesn't trust Mordred anymore.