St. Thomas Aquinas, in his Commentary on the Gospel of St. Matthew, writes on v. 17:9:
Afterwards, the command to delay the revelation of this vision is related; hence, he says, And as they came down from the mountain, Jesus charged them, saying: Tell the vision to no man. But what is the reason for this? There are three reasons. The first is that, as Jerome says, it was going to be that Christ would suffer and that the Jews would be scandalized; “Unto the Jews indeed a stumblingblock” (I Cor. 1, 23): wherefore, if they had heard this, they might have been more scandalized: hence, they would have reckoned Christ’s suffering to have been unimportant. Remigius expounds this verse as follows: it was because if He had made this vision known, He would never have accomplished what He desired to happen, and so, He would have thwarted His desire; for it is stated in Luke 22, 15: “With desire I have desired to eat this pasch with you.” Hilary expounds this passage as follows: He commanded silence because it was not fitting that spiritual glory be made known except through spiritual visions; but they were not yet spiritual; “As yet the Spirit was not given” (Jn. 7, 39).
The Catholic Haydock Commentary says:
Ver. 9. Tell the vision to no man, till the miracle of his resurrection has prepared the minds of men for the belief of this. Expose not an event so wonderful to the rash censure of the envious Pharisees, who calumniate and misrepresent my most evident miracles. Jesus Christ also gave a lesson here to his followers to observe the closest secrecy in all spiritual graces and favors.