The same reason he told Abraham to offer his only son in sacrifice, but never intended it to happen.You never answered the question as to why God would make such an empty threat if He was never going to fulfill it. What does that say about God?
To humble the man and exalt God.
The so called "empty" threat of God to Moses was made in order that Moses would be humbled. As a servant should be humbled. Yes, God knew the reaction and prayer of Moses. Still he issued this "threat."
God's intentions were, are and have always been to bring forth the nation of Israel from the seed of Abraham. Moses prayed but did not remind God of this, nor did he change the will and purpose of God. The entire incident was provided and designed to humble Moses and teach us humility. That we would pray according to His will and not our own.
The everlasting covenant has ever always been before God.
"Nevertheless I will remember my covenant with thee in the days of thy youth, and I will establish unto thee an everlasting covenant. Then thou shalt remember thy ways, and be ashamed, when thou shalt receive thy sisters, thine elder and thy younger: and I will give them unto thee for daughters, but not by thy covenant."
Ez. 16.60-61
God Bless you in your walk and service to Him.
Bro. Dallas