The answer is:

Because this is a situation calling for the acknowledgment that whatever evil has befallen me is due to my sin. Sin is the root of the evil, while the aim is to remove the harm; seeking forgiveness is a secondary aim. He therefore did not explicitly ask for the harm to be removed, conscious that he had done wrong and acted unjustly, and that he himself had brought the harm upon himself. It was thus fitting for him to acknowledge his wrongdoing, thereby addressing what would remove its cause. Nor did he explicitly ask for forgiveness, because this is a secondary aim for the distressed servant, whereas relief from distress, while it is present, is his primary aim. By its nature, the soul seeks the relief it needs from present harm before seeking, as a secondary aim, deliverance from harm it fears may occur in the future. The primary aim in this situation is forgiveness and seeking relief from harm. This therefore takes precedence in his intention and desire, and the most effective means of attaining it is to remove its cause. Thus, he expressed what would achieve his aim.

This becomes clear through a discussion of his words:

“Glory be to You.”

This expression entails magnifying the Lord and declaring Him free of imperfection. Here, the situation calls for declaring Him free of injustice and of punishing someone without sin. He is saying: “You are holy and far above wronging me or punishing me without sin. Rather, I am the wrongdoer who has wronged himself.”

Allah Most High says:

“We did not wrong them, but they used to wrong themselves.”

Allah Most High also says:

“We did not wrong them, but they wronged themselves.”

And He says:

“We did not wrong them, but it was they who were the wrongdoers.”

And Adam (peace be upon him) said:

“Our Lord, we have wronged ourselves.”