This is the decree of God. The Father, the Son and the Holy Spirit, one God in the eternal counsel decreed that this is how it would be.
People naturally ask the question – Why cannot the Holy Spirit come until Christ has gone? Why does Christ have to go before the Holy Spirit may come? And the answer is simple. The whole purpose of the Holy Spirit’s coming will be to apply the work of Christ. So the work of Christ must be carried out before the Holy Spirit comes. Christ must go to Calvary’s cross, and suffer and die and make an atonement for the sins of all the redeemed. Christ must rise from the dead and ascend into heaven. His great atoning, intercessory, mediatorial work must be accomplished because the purpose of the coming of the Holy Spirit is to apply that to the hearts of men and women. So naturally he says, ‘If I go not away,’ meaning if I have not accomplished all that I have come to do and then ‘I go not away, the Comforter will not come unto you.’