New Jerusalem is a city in resurrection and therefore is incorruptible. At present corruption is all around us. There is physical corruption, such as foods spoiling, and there is moral corruption, unrighteous behavior.
Some people strive to reduce or eliminate moral corruption, believing that men can behave properly through some combination of exhortation, education, laws, and public pressure. While this striving is good, it ignores a fundamental fact stated in Romans 5:
• through the disobedience of one man the many were constituted sinners (v. 19)
• through one man sin entered into the world, and through sin, death; and thus death passed on to all men (v. 12)
Because sin and death are in all men, a righteous, incorrupt society is impossible in this age. The efforts of those who strive for righteousness probably help reduce unrighteousness/corruption but cannot eliminate it.
The answer is Jesus Christ. This answer includes many steps. First are eternal facts completed without our involvement: God came in the man Jesus, He lived a perfect human life, He died for the sins of us all, and He rose to impart incorruptible life into us.
Then there are facts involving us, as in 1 Peter 1:23—everyone who believes in Jesus Christ is “regenerated not of corruptible seed but of incorruptible, through the living and abiding word of God.” When we were born again, we received an incorruptible life.
Now we face a choice moment by moment—to live by our natural, corrupted life or to live by our new, uncorrupted life. One Biblical expression of this choice is Romans 8:6, “the mind set on the flesh is death, but the mind set on the spirit is life and peace.” We face either death with its corruption or life with its incorruption. Because of this choice, at present Christians are not always perfect.
Gradually, as we cooperate, the incorruptible life grows in us. This process in our present life plus our transfiguration at the Lord’s return will bring forth New Jerusalem incorruptible and fully in resurrection.