I think God had several reasons for this:
1. It shows us how truly sinful mankind is.
Adam and Eve had everything going for them, and they blew it. The Israelites had God's direct revelation. God sent them prophets. And they still walked away from Him. Through their failures we see ourselves - just as sinful as they were.
2. It shows God's character.
God's holiness is reflected in His no-tolerance policy of sin.
God's justice is reflected in the punishment He metes out.
God's patience is reflected in His long-suffering dealings with Israel.
God's love is reflected in His reaching out to His people again and again.
God's faithfulness is reflected in His fulfillment of His promises.
3. The Old Testament sets us up to understand the New Testament.
The Messiah's arrival does not happen in a vacuum. It happens against the backdrop of a people seeking hope in a hopeless situation. And today, this is still the case. Individuals today still only come to Christ once they recognize the futility of their efforts and their need for a Savior.
So what does this mean for us as we approach the task of teaching the Bible? Of telling others about God's plan and the hope they can find? We would do well to follow God's outline. Tell the story cover to cover. Start at the beginning. Maybe don't get bogged down in too many details. However, don't forget to tell the whole story. It's the greatest one mankind has ever known.