Why hasn't the end happened?

“I tell you the truth, some who are standing here will not taste death before they see the kingdom of God come with power.” Mark 9:1

“tell you the truth, this generation will certainly not pass away until all these things have happened” Mark 13:30

Why did the last day not happen in the apostles’ lifetime? It seems to me as though the gospel writers thought that Jesus thought the end of time would occur shortly after his ascent into heaven- less than a generation after. Why didn’t it happen?

The passages you cite to not necessarily refer to “the end.” Mark 9:1 says simply that the present generation would see “the kingdom of God has come in power.” The kingdom of God did come in the person of Jesus Christ through his death and resurrection. The establishment of the Church was understood by early Christians to be the beginning of the kingdom of God on earth.
Another understanding of that passage is that it was fulfilled by the apostles witness of the transfiguration of Jesus (Mark 9:2-8).

Mark 13:30 is a little more complex to interpret. It refers not just to the end but the beginning of the end, so to speak. Mark 13 describes in pretty good detail the coming persecution of Christians and the destruction of Jerusalem, all of which happened within the lifetime of some of those who heard Jesus speak those words. Jesus states when all those things happen that the end is “near” but that “of that day or hour, no one knows, neither the angels in heaven, nor the Son, but only the Father” (Mark 13:32).

It can appear at first glance that these passages are predictions of an immediate apocalypse. However when read in full context of the verses around them there seems to be a balancing act being portrayed. In a certain sense Jesus is proclaiming a military expression: “hurry up and wait.” Jesus is preaching that we need to always be prepared to give an account of our lives and our faith. We do not know when we will face either persecution from human authority or the divine judgement and so we must live our faith prepared for either outcome.

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