Behold, we are going up to Jerusalem; and the Son of man will be delivered to the chief priests and scribes, and they will condemn him to death, and deliver him to the Gentiles to be mocked and scourged and crucified, and he will be raised on the third day. Matt. 20:18, 19, RSV.
They don't know it, but the disciples are totally unplugged when it comes to the essence of the kingdom and what was happening to Jesus. They were having visions of thrones (Matt. 19:27, 28) and personal greatness (Matt. 20:20-24) while Jesus struggles with forebodings of a cross and rejection. The disciples and Jesus are operating in two different worlds.
As a result, He supplies them more of what they need to hear rather than what they want to hear. This time it is a third replay of His predictions about His forthcoming death and resurrection. That is their greatest need at the moment. After all, they are heading for Jerusalem and the cross of Calvary.
Jesus had first raised the topic immediately after Peter's confession that He was the Christ. They had at last identified Him as the Messiah, but they had no idea of what that meant. So Jesus first tells them of His forthcoming death and resurrection in Matthew 16:21. He returned to the topic in Matthew 17:22, 23. And now in Matthew 20:18, 29 He sets it forth again. But with each repetition He fills in more details. Now He informs the disciples that the Jewish leaders, after condemning Him to death, will "deliver him to the Gentiles." Other first-time details are the manner of His suffering--mocking and flogging--and the type of His death--crucifixion. "The effect," pens R. T. France, "is to emphasize not only the totality of the rejection (Jewish leaders and Gentiles), but also the humiliating and the harrowing pain; this is to be no glorious martyrdom, but an ugly, sordid butchery. It is thus all the more striking to read yet again here that he will be raised on the third day."
Yet the disciples heard none of it! They had their own concerns that were blocking them from grasping the clear word of God to them. Their own ambitions and daily struggles made them deaf to Jesus.
Here is a theme of the gospel story. And rightly so. A perverseness of human nature makes us alive to our self and our desires and dead to God and His will. And that, my friends, is not merely a first-century phenomenon.