Salvation Needed
If humanity had not defected we would in due course have ripened, and God would then have become a man to raise us to union with himself.
We did defect, and as a result we have generally lived badly. We have neglected development and have violated natural principles, making ripening impossible.
After our defection God began to nudge certain humans back onto the right track. He called ancient Israel to be the prime vehicle for this work. Some supposed that Israel would lead humanity to ripeness. But this was not happening. Why not?
Wrongdoers' hindrance.
So why did God not simply realign everyone? He has the power to achieve all his purposes, but not always in the way he would prefer. Realignment is not like creation "out of nothing"; he works with the material our folly has left him. Probably he has realigned as many as possible.
Welldoers' residual bad habits.
Even in Israel God had foes. The prophets of Israel recognised this. They hoped for a "new covenant" in which the whole nation would be God's friends and his friends' ways would be more fully transformed.
I remember my affliction and my wanderings, the wormwood and the gall. My soul continually remembers it and is bowed down within me. But this I call to mind, and therefore I have hope: the steadfast love of Yahweh never ceases; his mercies never come to an end. They are new every morning; great is thy faithfulness.
Lamentations 3.19-23.
"The days are coming, says Yahweh, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their ancestors ..., a covenant that they broke .... I will put my law within them, and write it on their hearts."
Jeremiah 31.31-34.
And some of the prophets of Israel saw beyond Israel.
"It is too light a thing that you should be my servant to raise up the tribes of Jacob and to restore the preserved of Israel; I will give you as a light to the nations, that my salvation may reach to the end of the earth."
Isaiah 49.6. (The prophet envisages God addressing a future leader of Israel.)
"It is implausible that the cure of humanity's ills should depend on one man, as Isaiah envisaged."
This is the "folly to the nations" that Paul of Tarsus mentions (1st Corinthians 1.23), known as the scandal of particularity. But it only seems foolish to those who have already rejected this Creed's view of human nature, both as originally created and as it has become; to those who see their need for union with God, and especially to those who see the plight of humanity as incorrigible wrongdoers, it all makes sense.
"If the need were as you say, God would have saved earlier instead of waiting for so long."
God did not passively await the time. He actively prepared the world, and came among us "when the time had fully come" (Paul of Tarsus, Galatians 4:4).
The following is conjecture, and there may have been other factors that we shall never understand.
While all other nations assumed that man's desires were either good or inevitable, Israel developed an awareness of the dire consequences of human wickedness and of God's intention to cure it. Thus God created a mental climate suitable for understand the significance of Christ.
By the time of Christ, Israel's education was complete, in that as a nation they had finally settled into a stable recognition that "God is one", the last words of many who were killed for refusing to accept polytheism. Meanwhile the Roman Empire made international communications easier than ever before, and the prevailing culture was in crisis, receptive to new ideas.