Here at the outset, understand that for Mark’s Gospel the backstory is the story of Israel. But many modern Christians aren’t aware of this. We do not know why things were the way they were when we read this Gospel. N.T. Wright notes most of us, if we are not careful, see the Gospels in general,
“[S}imply as God’s answer to the plight of the human race in general. The backstory most of us live with hasn’t been the story of Abraham, of Moses, of David, of the prophets; it’s been the story of Adam and Eve, of “Everyman,” sinning and dying and needing to be redeemed. Israel’s story sneaks in alongside or is to be quietly left aside. It seems, after all, to be so dark— such a failure, such a disappointment. Or God’s sort of first but failed attempt to save humanity from sin."
But Israel’s backstory is so important to Mark (and the other Gospels, for that matter). As the early church came to terms with who Jesus was, they realized they could not tell his story without Israel’s story.