Summary: Why did Jesus Come when He did. Why not before? Why not later? We will see that God had just the right time in history, like none other, for Jesus to come.
Why 2 thousand years ago? Think about it, the land of Israel was occupied by the Romans. A pagan nation ruled over Israel and the Jewish people were in a state of upheaval, and the priestly system was corrupt. If Jesus came as a Ruler, or as a King, as a warrior Messiah, to drive out the Romans, this would make sense. But Jesus came as Savior. He came, born to poor parents, became a teacher and healer, only to die on a Roman cross. Jesus came as Savior, to die for our sins.
In his role of Savior, was there a better time He could have come?
Why not before the flood?
Why not before the exile and destruction of Jerusalem (around 580 BC).
Why not 600 AD before Mohammad came on the scene?
Why not before, during, or after the dark ages?
What was special about that day and age when Jesus was born?
From our vantage point of being 2,000 years removed and we can look back into history, we can see all the historical and cultural factors in place for the very perfect time in all of history for Jesus to come. Our emphasis is on Galatians 4:4, but to place the passage in context we will read verse 3-5.
Galatians 4:3–5 (NKJV) 3 Even so we, when we were children, were in bondage under the elements of the world. 4 But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law, 5 to redeem those who were under the law, that we might receive the adoption as sons.
You have probably heard the phrase many times, "timing is everything". There is a great deal of truth in that statement.
The difference between a good joke and a bad one is a person’s sense of timing. An appropriate pause makes a joke, an inappropriate pause can kill the same joke.
Timing is essential when dealing with people. You don’t ask for a raise when business is not going well or when things are tense around the office. You don’t try to correct with someone who feels threatened by you. You don’t ask for a favor when someone is under a lot of stress or angry.
Timing is important in cooking. The juicy steak on the grill is raw meat if cooked for too little time and a clump of charcoal if it is cooked too long.
Timing is important in medicine. If you catch a problem early you will be able to treat it more effectively.
Your timing is important in taking medication. If you take your medicine as directed it will be helpful. If you skip doses, it loses its effectiveness. If you take extra doses it can be deadly.
Timing is important in finance. When you invest in a particular stock and when you sell the same stock will make the difference between whether you make money or lose it. Knowing when to borrow and when not to borrow is the key to financial independence.
Timing is important in your spiritual life as well. Jesus was very conscious of timing. He lived His life with an acute awareness of God’s timing for His life. The gospel of John records these words of Jesus: 
John 2:4 (NKJV) Jesus said to her, “Woman, what does your concern have to do with Me? My hour has not yet come.”
Jesus says this to His mother at the Wedding at Cana. Jesus says the following to His brothers who urged Jesus to show himself to the world (and they did not believe in Him at that point in time either).
John 7:6–8 (NKJV) Then Jesus said to them, “My time has not yet come, but your time is always ready. 7 The world cannot hate you, but it hates Me because I testify of it that its works are evil. 8 You go up to this feast. I am not yet going up to this feast, for My time has not yet fully come.”
Jesus avoids those who would have him killed.
John 8:20 (NKJV) These words Jesus spoke in the treasury, as He taught in the temple; and no one laid hands on Him, for His hour had not yet come. Even in the Old Testament,
God waits judgement until the timing is right. God was explaining to Abraham about the time for future generations of his family.
Genesis 15:16 (NKJV) But in the fourth generation they shall return here, for the iniquity of the Amorites is not yet complete.”
But Jesus’ hour did come. After the triumphal entry, during the preparation for the Passover just a few days before His crucifixion, Jesus announced:
John 12:23 (NKJV) But Jesus answered them, saying, “The hour has come that the Son of Man should be glorified.