Why in heaven’s name would Jesus do that? They’d just had the spiritual experience of their lives. They had just seen God’s glory reflected in their rabbi. In light of Peter’s confession that Jesus was the Messiah, it was as reasonable as they could imagine – if one could describe as reasonable an experience they could not have imagined before it actually happened. This was, in a way, the ultimate confirmation of the time they had spent with Jesus. They had been with him almost three years. They had left it all to follow Jesus, friends and family and good careers, and now they had the confirming experience. God had show unmistakably, unequivocally, that his favor shone on Jesus. There he was, in light they could hardly bear, flanked by the heroes of their faith, Moses and Elijah. And after all that, Jesus says, "Tell no one about the vision until after the Son of Man has been raised from the dead." How could he do that?
Perhaps it was to prevent them becoming, as folks say, “too heavenly minded to be any earthly good.” If they spent all their time looking in awe at Jesus, they might well miss all that went on around him – and from this point, there’s still a lot to see around Jesus.
Perhaps it was because of the distance they still had to travel. The journey from the foot of the Mount of the Transfiguration leads up to Jerusalem – and to the cross. The story would not end, and could not be understood, if they never left the mountain, even in their own minds.
Perhaps it was because they hadn’t yet understood their vocations, vocations that would not actually be set until after the resurrection. Indeed, as the story could not be understood if they never left the mountain, neither could their ministries take form if they never left the mountain.
One way or another, they had much yet to see, far yet to go, and a lot yet to learn. For all the excitement they had experienced – and for all of the joy found in simply adoration – there was more to experience; and they would never see it if they stayed, even in their own minds, up on the mountain.