Everything Had to be Made Subject to Christ
The New Testament says that before he was exalted, he was beneath the angels. But after God raised Jesus from the dead, God subjected all things to Jesus, and now the only thing not subjected to Christ is God himself.
And He (God) put all things in subjection under His (Jesus) feet, and gave Him (Jesus) as head over all things to the church, which is His body.
For “God has put all things in subjection under his feet.” But when it says, “all things are put in subjection,” it is plain that he is excepted who put all things in subjection under him.
Yahweh says to my lord: “Sit at my right hand until I make your enemies a footstool for your feet.”
but we do see Jesus, who for a little while was made lower than the angels, now crowned with glory and honor because of the suffering of death, so that by the grace of God he might taste death for everyone.
Therefore let all Israel be assured of this: GOD has made this Jesus, whom you crucified, both Lord and Messiah!
Jesus Hands the Kingdom over to God
If Jesus is God himself coming to rule the world in the Kingdom, why would he hand the Kingdom over to God in a final act of submission so that "God" could rule over everything including Jesus?
"then comes the end, when He hands over the kingdom to the God and Father, when He has abolished all rule and all authority and power. For He must reign until He has put all His enemies under His feet. The last enemy that will be abolished is death. For “He has put all things in subjection under His feet”. But when He says, “All things are put in subjection,” it is evident that He is excepted who put all things in subjection to Him. When all things are subjected to Him, then the Son Himself also will be subjected to the One who subjected all things to Him, so that God may be all in all."
The above post is based on an article taken from here.