Why didn't God send Jesus during Noah's time instead of The Flood? For God so loved the world, that he gave his only begotten Son, that
  whosoever believeth in him should not perish, but have everlasting
  life. - John 3:16


John 3:16 tells us that God loved us, so He sent Jesus to save us.

So why didn't He do this during Noah's time?
It's rather ironic — the thrust of your question is "Why did God wait so long?" Andrew Lloyd Webber asks the exact opposite question — "Why come so early?"  As Judas sings in the finale:


  Every time I look at you
  I don't understand
  Why you let the things you did
  Get so out of hand
  You'd have managed better
  If you'd had it planned
  Now why'd you choose such a backward time
  And such a strange land?
  If you'd come today
  You could have reached the whole nation
  Israel in 4 BC had no mass communication


The truth, however, is that God in his timing knew exactly when to come.  As Paul says in Romans 5:6


  For while we were still weak, at the right time Christ died for the ungodly. 


That phrase "at the right time" is a single Greek word, kairos — which is an opportune moment, the perfect time for a thing.  Paul says much the same thing in Galatians 4:


  3 So also, when we were underage, we were in slavery under the elemental spiritual forces[a] of the world. 4 But when the set time had fully come, God sent his Son, born of a woman, born under the law, 5 to redeem those under the law, that we might receive adoption to sonship.


Why was the early 30s the perfect moment? Why was the fullness of time not earlier? Only God knows. We can speculate, but the truth is, only God knows.

One thing we do know is that God did not desire multiple sacrifices. Rather, he wanted one "for all time," as Hebrews states in chapter 10:


  And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all. 11Day after day every priest stands and performs his religious duties; again and again he offers the same sacrifices, which can never take away sins. 12But when this priest had offered for all time one sacrifice for sins, he sat down at the right hand of God. 13Since that time he waits for his enemies to be made his footstool, 14because by one sacrifice he has made perfect forever those who are being made holy.


Given how controversial literal interpretations of anything prior the written record, it is not overly surprising to me that God would wish to wait, for our benefit, to come to a time when written records would be preserved. (Heck, theologians even discounted the existence of a real King David until just recently when, surprise, written evidence found.)

Had Jesus come in antediluvian times, Jesus would have been hopelessly mythological.  (Again, even with as much historical record as does exist&nbsp;— Tacitus, Josephus, etc.&nbsp;— many still regard Jesus as myth!)

And, absent the flood that did come, it is also equally likely that, frankly, God would look like too much of a softy.  I mean, think about it&nbsp;— in Genesis, God is forever trying not to blow people away.  He withholds the death that Adam &amp; Eve deserve.  He withholds the death that Cain deserves. And after the Flood at Babel, God disperses the people by the least intrusive means possible&nbsp;— confusing their language.

You can argue that God is not just (although I'd argue back with this question). You can argue God is too lenient.  But the truth is, God has exactly the timing that he desires.