Why did God decided not to destroy the world with flood again?
In Genesis 6, God saw that which he had created, mankind, had turned against the Creator. It was this that saddened Him. "And GOD saw that the wickedness of man [was] great in the earth, and [that] every imagination of the thoughts of his heart [was] only evil continually. And it repented the LORD that he had made man on the earth, and it grieved him at his heart. And the LORD said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them." (Gen. 6:5-8) God knew that if man was allowed to continue, that they would never recover which would not have paved any way for His Son to come to the earth and to die on the cross for the sins of the world which was first prophesied in Genesis 3:15 which says, "And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel." So, God had destroyed all that was in the earth by water, except the eight souls and those animals inside the ark. He had cleansed the world and showed by implication the effectiveness of God's cleansing ability.
Later, this same kind of cleaning would be demonstrated in baptism by which the blood of Christ cleanses the sinful soul clean when they, by obedience, go down into the water by immersion. Now, in Genesis 9 God said to Noah, "And I will establish my covenant with you; neither shall all flesh be cut off any more by the waters of a flood; neither shall there any more be a flood to destroy the earth. And God said, This [is] the token of the covenant which I make between me and you and every living creature that [is] with you, for perpetual generations." God had given the world another chance to make way the path of the Lord. We see several instance as before how God is frequently trying to guide the people with people such as Moses, the prophets and later of course, His Son, Jesus Christ who was God in the flesh (Jn. 1:14). And it was Christ who is able to wash away the sins of those who came to Him in obedience. Now, God has promised to give man this opportunity to follow Him.
Therefore, he continues to be longsuffering. For, "The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance." (2 Pet. 3:9). God's desire is that he wants all mankind (every Male and female) to obey his commands. However, since God has given mankind, ". . . all things that [pertain] unto life and godliness, through the knowledge of him that hath called us to glory and virtue:" (2 Pet. 1:3), God has given mankind His best. Therefore, no longer will he restart the population of the earth by a world-wide flood in order to give them another chance. He simply will in, "the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up." (2 Pet. 3:10) This is why Christians, "who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels" (2 Thess. 1:7). One day the end will be here and judgment will be upon those who thought that they would have another chance to redeem themselves (like another flood). However, "The Son of man shall send forth his angels, and they shall gather out of his kingdom all things that offend, and them which do iniquity; And shall cast them into a furnace of fire: there shall be wailing and gnashing of teeth. Then shall the righteous shine forth as the sun in the kingdom of their Father. Who hath ears to hear, let him hear." (Matt. 13:41-43).