In view of Mark 9:1 why say the kingdom of God will come at Jesus' second coming?

It cannot be the Transfiguration. The context is in the preceding verse, such as in Mark 8:38 “When He comes in His Father’s glory with the holy angels”, Luke 9:26 “When He comes in His glory and in the glory of the Father and of the holy angels,” and Matthew 16:28 “For the Son of Man will come in His Father’s glory with His angels, and then He will repay each one according to what he has done.” All this makes it clear that JC was referring to the Second Coming, not the Transfiguration. In each of the gospels where JC makes this prediction, he is clearly referring to the events of the Second Coming, not a mere transfiguration, as the context clearly shows in each passage: The kingdom is to come in power, that is with an army of angels and judgement time, not just some transfiguration on a hill nearby. He would not bother saying “some standing here will not pass away”, and especially “this generation will not pass away” if he was referring to an event that took place a mere six or eight days later for the benefit of his three closest disciples that he knew personally. And yes, all of these disciples and that entire generation did pass away before the Second Coming, the kingdom of god coming in power.

To understand this verse you must have a clear understanding of what "The Kingdom of God" actually is. A King's kingdom is where he is obeyed completely. His rule and reign are complete. So when Jesus uses this term it can easily be misinterpreted to mean a coming "thing" or heaven or some other such distant and more nebulous unexplained item.

The more scripturally congruent interpretation is the one above. Using those terms it is easy to see in Matthew 6:10

Your kingdom come, your will be done, on earth as it is in heaven.

or in Matthew 6:33
But seek first the kingdom of God and his righteousness, and all these things will be added to you.

Here in Matthew 7:21

Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven.

shows that entering the kingdom of God is not possible without repentance, which is also congruent with Luke 13:3 and 13:5

No, I tell you; but unless you repent, you will all likewise perish.

In Matthew 13:45-46 there is further congruence

Again, the kingdom of heaven is like a merchant in search of fine pearls, who, on finding one pearl of great value, went and sold all that he had and bought it.

Clear after you apply the correct logical premise by comparing it to related passages in scripture.

That might depend on the definition of "the Kingdom of God" as well as where an individual stands in relation to it.

First, the O.T. shows that God's kingdom is an everlasting one, as Daniel's dream of the four beasts shows. The relevant part is from verse 13:

"Behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him. And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages, should serve him: his dominion is an everlasting dominion, which shall not pass away, and his kingdom that which shall not be destroyed." (Daniel 7:13-14)

There is no end to it, and Christ is the King of this Kingdom. He has subjects who acknowledge him as their heavenly King. It is at least global, but scripture goes on to show it is also cosmic in scope. The King is Sovereign, not only in title but in exercise of his authority. The last book of the Bible resonates strongly with Daniel's prophetic visions, especially in chapters 21 and 22.

But Mark knew about the Kingdom of God on earth, as represented by the nation of Israel and its line of kings, representing the Great King, who would one day appear, sovereignly ruling all the nations. Mark recognized Jesus as that Messianic King. This means that those familiar with the scriptures could view the Kingdom of God in a three-fold sense; As having had visible representation through the nation of Israel on earth i.e. earthly; As having visible continuation through the incarnate King of the Kingdom of God - Christ i.e. heavenly; As having future fulfilment in completion of its final scope i.e. cosmic.

This means that for O.T. believers, they had the earthly rulers of Israel as their limited vision. For N.T. believers, they had the vision of the appearing of the Son of man in their midst, the divine King, as at his transfiguration. For believers after Christ's resurrection, they had the understanding of the revealed mystery of God - Christ in them, the hope of glory (Col. 1:27). By the time the last book of the N.T. had been written, they understood what Jesus meant when he had said, “Truly I tell you, some who are standing here will not taste death before they see that the kingdom of God has come with power.” Just read that last book, and you can see the King in his glory, ruling from heaven, bringing in the Day of Resurrection and Judgment, all usurpers to his sovereign rights exposed, ousted, and suffering everlasting damnation. The whole cosmos will be restored to its original, pristine perfection, with the Kingdom of God over everything, and everyone in existence.

The Kingdom of God has never gone away. It's always been there. It's just sinners with closed eyes who can't even see it right under their noses (as when Jesus walked on earth, the King of that Kingdom), nor can they find entrance into it even when they are not far from it unless they humble themselves to be like little children (Matthew 18:1-4).

The Kingdom of God becomes a reality to people while they are alive on earth when they bend their knee repentantly, faithfully, in Jesus' name, confessing him King and Saviour. They continue as subjects of his Kingdom, death unable to end their place in the Kingdom. Then, at the appointed hour, the King will spectacularly return as King to break the nations with his rod of iron, all his faithful subjects accompanying him. So, the answer to your question is that that is the sense in which the Kingdom of God comes, in a particular way, at that particular time. It becomes apparent to all who refused to acknowledge the reality of it, or that Christ is its King - too late for them, of course. But for all believers in the O.T. and the N.T., they have always had a gradually revealed understanding of the scope of the Kingdom of God, enjoying something of its blessings even while on earth. It's just that, when all enemies of the Kingdom have been finally dealt with by the King (Matthew 21:33-44) the immensity of that glorious Kingdom will be acclaimed by all, who will all be willing subjects of it.