Why didn’t Jesus come to earth sooner? John Piper answered this question in an episode of Ask Pastor John by explaining how the length of Israel’s long history before the coming of Jesus acts as “a lesson book for the nations about the Gospel”:
The thread of Old Testament history runs from creation through fall into sin through election of Israel, thorough exodus, through giving of the law, and then a thousand years, give or take, of the inability of Israel to keep the law. A thousand years of covenant-breaking over and over again. It is a bleak, bleak history as you read it....
Now in Romans 3:9–18, Paul quotes half a dozen passages from the Old Testament, mainly from the Psalms, but he calls them “the law.” And the point of those passages is to show that everybody is under judgment—everybody, Jew and Gentile—are sinners. And then Paul says one of the most important sentences in the Bible regarding what God has been up to for these 2,000 years of Jewish history. Verse 19: “Now we know that whatever the law says”—and he is referring back to those statements about the failure of Israel and all people—“whatever the law says it speaks to those who are under the law”—that is Israel—“so that every mouth may be stopped, and the whole world may be held accountable to God. For by works of the law no human being will be justified in his sight” (Romans 3:19–20)....
In other words, the history of the Jewish people and the record of its failure to keep the covenant that God made, that record not only shows that Jews will never be able to save themselves, but the whole world is accountable because it, too, has failed in all of its ways of trying. Because, if the Jews can’t do it with this much special revelation and privileges as they have had, how much less will the world be able to work its way into God’s favor? And then he gives this in verse 20: for no human being will be justified in the sight of God.
Now that, it seems to me, is as close as we get in the Bible to an explanation of why there is such a long and contorted, disappointing, frustrating history. God ordained that the history of Israel would develop in such a way and for as long as it did, so that Israel would be a lesson book for the nations about the gospel, a lesson book for the nations that no one can be justified by law keeping. And evidently God believes that the lengthy story of failures under every condition of advantage makes the point in the best possible way.
Read or listen to the rest of his answer.