Why did Jesus say that unless he went away, the Comforter could not come?

Answer:

"Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you" (John 16:7).

Notice that in the verse there is a slight difference between what you asked and what Jesus said. Jesus did not say the Holy Spirit could not come, but that he would not come until Jesus went away.

Jesus is discussing with his disciples, on the night he would be betrayed, that shortly he would die, but then be resurrected and return to heaven. For the disciples, this would seem to be the end of all their hopes, but Jesus tells them that it was advantageous for them that he leaves.

Each of the Godhead had roles to fulfill in the salvation of mankind. Jesus had to complete his duties before the Spirit could begin his. The Spirit would be involved in the spreading of the Gospel message throughout the world "However, when He, the Spirit of truth, has come, He will guide you into all truth; for He will not speak on His own authority, but whatever He hears He will speak; and He will tell you things to come" (John 16:13). But there could be no good news to spread if Jesus did not both die and was resurrected.

"Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast that word which I preached to you--unless you believed in vain. For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, and that He was buried, and that He rose again the third day according to the Scriptures" (I Corinthians 15:1-4).

The fact that Jesus arose is a key cornerstone of the gospel message.

"Now if Christ is preached that He has been raised from the dead, how do some among you say that there is no resurrection of the dead? But if there is no resurrection of the dead, then Christ is not risen. And if Christ is not risen, then our preaching is empty and your faith is also empty. Yes, and we are found false witnesses of God, because we have testified of God that He raised up Christ, whom He did not raise up--if in fact the dead do not rise. For if the dead do not rise, then Christ is not risen. And if Christ is not risen, your faith is futile; you are still in your sins!" (I Corinthians 15:12-17).

Jesus laid the foundation upon which many in the world could be saved. But without that foundation, the work the Spirit needed to accomplish would have been useless. That is why it was to the disciples' and mankind's advantage that Jesus completes his work, which meant leaving the world so that the Spirit would be able to do his work. "In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory" (Ephesians 1:13-14).