Mat 17:9 Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
You should be asking Jesus, he said it was a vision, I agree with him. though I understand what the vision is showing, they are speaking of his decease and all that he was to accomplish in Jerusalem and how they (The law representive of Moses) and the prophets (representive of Elijah) in the vision spake concerning Christ, Peter wanted to "separate them" Gods voice spoke saying this is my Son hear ye him speaks to this scripture...
Mark 9:4 And there appeared unto them Elias with Moses: and they were talking with Jesus.
Mark 9:5-6 And Peter answered and said to Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias. For he wist not what to say; for they were sore afraid.
Mark 9:7 And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him.
Mark 9:8 And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves.
Which answers in this...
Romans 3:21 But now the righteousness of God without the law is manifested, being witnessed by the law and the prophets
Luke 24:34 These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
Thats why they werent to tell any man till he rose from the dead (as the effect of every vision was being fulfilled Jesus Christ) "they speaking of his decease"
1Peter 1:11 Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow.
Not unto themselves did they minister these things but unto us
Luke 24:25 Then he said unto them, O fools, and slow of heart to believe all that the prophets have spoken: Ought not Christ to have suffered these things, and to enter into his glory?
Luke 24:27 And beginning at Moses and all the prophets, he expounded unto them in all the scriptures the things concerning himself.
Heb 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds;
They had to wait to say anything because he is declared the Son of God by the spirit of holiness and by the resurrection of the dead. so showing Moses and Elijah vanishing and Christ alone standing there as the righteousness of God was manifesting (speaking of Christ) fulfilling those things which were witnessed by them (by both the law and the prophets)