Because God isn't a quitter. He had a perfect plan and finally came to forgive man from His sin without punishment but with love if man so chooses. I'm just answering your question...not trying to convert anyone.
If God was not a quitter, why punish everyone but Noah and his family? He could have asked ''saintly'' Noah to help the others become more saintly instead of wiping them out. And since the others were being punished for having ''sensual desires'', didn't Noah and his family also have sensual desires since they did ''procreate"? And the world has ALWAYS been corrupt and filled with violence, that wipeout has not solved anything. And why punish all the animals not on Noah's ark because of man's "immoral excesses"?
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Noah was a righteous man and walked with God. Seeing that the earth was corrupt and filled with violence, God instructed Noah to build an
ark in which he, his sons, and their wives, together with male and female of all living creatures, would be saved from the waters. Noah entered the ark in his six hundredth year, and on the 17th day of the second month of that year "the fountains of the Great Deep burst apart and the floodgates of heaven broke open" and rain fell for forty days and forty nights until the highest mountains were covered 15 cubits, and all earth-based life perished except Noah and those with him in the ark.
In Jewish legend, the kind of water that was pouring to the earth for forty days is not the common, but God bade each drop pass through
Hell of
Gehenna before it fell to earth, and the 'hot rain' scalded the skin of the sinners. The punishment that overtook them was befitting their crime. As their sensual desires had made them hot, and inflamed them to immoral excesses, so they were chastised by means of heated water.
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1896 illustration of the symbol of the rainbow, which God created as a sign of the covenant
After 150 days, "God remembered Noah ... and the waters subsided" until the ark rested on the mountains of Ararat. On the 27th day of the second month of Noah's six hundred and first year the earth was dry. Then Noah built an altar and made a sacrifice, and God made a covenant with Noah that man would be allowed to eat every living thing but not its blood, and that God would never again destroy all life by a flood.