AThere are several passages that show clearly that the sending of the Holy Spirit was consequent on the glorification of Jesus at God’s right hand:

“… the Spirit was not yet given, because Jesus had not yet been glorified” (John 7:39)

“This Jesus has God raised up, whereof all we are witnesses. Having therefore been exalted by the right hand of God, and having received of the Father the promise of the Holy Spirit, he has poured out this which ye behold and hear.” (Acts 2:32-33)

It is clear that the Holy Spirit could not have been sent before Jesus was glorified, but this does answer the question of why. Acts 2:32-33 gives us an indication that the sending of the Holy Spirit, called “the promise of the Spirit”, is connected with Christ’s exaltation. It would seem that the gift of the Spirit was in response to the Father’s satisfaction with the work of the cross (John 13:31-22). In other words, the Father was so delighted with the finished work of His Beloved Son, that He gave to Christ the promise of the Spirit, which He then sent down to the earth.

Another point also might help us with this question. That is, the unity that was formed when the Holy Spirit was sent down on the Day of Pentecost was the union of believers one to another as members of the body of Christ, and the union of the members to their Head in heaven (Christ Himself). See 1 Cor. 12:12-13, Col. 1:18; 2:19. It was fitting then that before the body was formed on earth, the Head would be glorified in heaven!