Why did God choose to do it all this way? I don't know. I'm just laying out what I understand to be the big picture shown in the Bible. Here are some thoughts.
1. We humans think our sin isn't such a big deal. We think we're basically pretty good blokes.
2. In the OT, through wrath and judgment (the flood, plagues, etc.) God showed that He sees our sin as HORRENDOUS
3. In the OT, God also stressed grace, as shown in his call of the patriarchs and his continual sparing of Israel despite her continual rebellion
4. In the OT, God gave the Law, in part to restrain sin externally through carrots and sticks, just as we do with young children (Prov. 22:15)
5. The main purpose of the Law, though, was to show us how sinful we really are and to prove to us our need for spiritual rebirth to change us internally, not just externally:
-- The Law aroused the rebellion within us and showed us that we are "utterly sinful." (Romans 7:13)
-- The unregenerate person can will what is right but cannot do it (Rom. 7:24)
-- The Law was therefore our schoolmaster to show us our need for Christ (Gal. 3:21-24)
6. In the NT, when God decided the time was right, he sent Christ to provide the way to forgiveness and spiritual rebirth (Gal. 4:4)
-- The leaders of Israel didn't understand that they needed to have Christ's righteousness given to them; they tried to establish their own righteousness through the law, something it was never intended to be used for (Romans 10)
-- Jesus told Nicodemus, a ruler of Israel, that he should not be surprised to learn that he must be reborn spiritually (John 3:7) [That was one of the main things their OT experience with the Law was supposed to teach them, but they didn't get it.]
7. Even in the NT we see that God isn't finished with his wrath against sin (Col. 3:6; most of Revelation)
8. We are now in a period where God is forebearing to give us time to repent until the full number comes in (Rom. 4; 11:5; 2 Peter 3:9)
Some Scriptures Cited Above
Romans 7:12-13: "So then, the law is holy, and the commandment is holy, righteous and good. 13 Did that which is good, then, become death to me? By no means! But in order that sin might be recognized as sin, it produced death in me through what was good, so that through the commandment sin might become utterly sinful."
Galatians 3:21-24: "Is the law, therefore, opposed to the promises of God? Absolutely not! For if a law had been given that could impart life, then righteousness would certainly have come by the law. 22 But the Scripture declares that the whole world is a prisoner of sin, so that what was promised, being given through faith in Jesus Christ, might be given to those who believe. 23 Before this faith came, we were held prisoners by the law, locked up until faith should be revealed. 24 So the law was put in charge to lead us to Christ that we might be justified by faith."
Galatians 4:1 What I am saying is that as long as the heir is a child, he is no different from a slave, although he owns the whole estate.
2 He is subject to guardians and trustees until the time set by his father. 3 So also, when we were children, we were in slavery under the basic principles of the world. 4 But when the time had fully come, God sent his Son, born of a woman, born under law, 5 to redeem those under law, that we might receive the full rights of sons.
1 Cor. 10:11 These things happened to them as examples and were written down as warnings for us, on whom the fulfillment of the ages has come.