Because His kingdom is not of this world.
===========
MT. 16:28 I tell you the truth, some who are standing here will not taste death before they see the Son of Man coming in his kingdom.
MT. 17:1 [NEXT VERSE] After six days Jesus took with him Peter, James and John the brother of James, and led them up a high mountain by themselves. 2 There he was transfigured before them. His face shone like the sun, and his clothes became as white as the light. 3 Just then there appeared before them Moses and Elijah, talking with Jesus...
9 As they were coming down the mountain, Jesus instructed them, Dont tell anyone what you have seen, until the Son of Man has been raised from the dead.
NIV Matthew 16:28
28. I tell you the truth, some who are standing here will not taste death beforethey see the Son of Man coming in his kingdom."NIV Mark 9:1
1. And he said to them, "I tell you the truth, some who are standing here will not taste death beforethey see the kingdom of God come with power."NIV Luke 9:27
27. I tell you the truth, some who are standing here will not taste death beforethey see the kingdom of God."
NIV John 8:51-53
51. I tell you the truth, if anyone keeps my word, he will never see death."
52. At this the Jews exclaimed, "Now we know that you are demon-possessed! Abraham died and so did the prophets, yet you say that if anyone keeps your word, he will never taste death.
53. Are you greater than our father Abraham? He died, and so did the prophets. Who do you think you are?"NIV Hebrews 2:9
9. But we see Jesus, who was made a little lower than the angels, now crowned with glory and honor because he suffered death, so that by the grace of God he might taste death for everyone.
John 21:20-24
20. Peter turned and saw that the disciple whom Jesus loved was following them. (This was the one who had leaned back against Jesus at the supper and had said, "Lord, who is going to betray you?")
21. When Peter saw him, he asked, "Lord, what about him?"
22. Jesus answered, "If I want him to remain alive until I return, what is that to you? You must follow me."
23. Because of this, the rumor spread among the brothers that this disciple would not die. But Jesus did not say that he would not die; he only said, "If I want him to remain alive until I return, what is that to you?"
24. This is the disciple who testifies to these things and who wrote them down. We know that his testimony is true.