Starting in 3 Nephi 2, the people begin to be very wicked, even after they see the sign of Christ’s birth! A few chapters later, they repent, but soon, most of the people go wicked again. There are lots and lots of wars, contentions, robbing, murders, etcetera, and etcetera.
But maybe there’s a point to all this. If they weren’t righteous enough to stay believers after they saw an amazing sign, then perhaps they weren’t righteous enough to see Christ.
And behold, that great city Moronihah have I covered with earth, and the inhabitants thereof, to hide their iniquities and their abominations from before my face, that the blood of the prophets and the saints shall not come any more unto me against them.
Yea, and the city of Onihah and the inhabitants thereof, and the city of Mocum and the inhabitants thereof, and the city of Jerusalem and the inhabitants thereof; and waters have I caused to come up in the stead thereof, to hide their wickedness and abominations from before my face, that the blood of the prophets and the saints shall not come up any more unto me against them.
And behold, the city of Gadiandi, and the city of Gadiomnah, and the city of Jacob, and the city of Gimgimno, all these have I caused to be sunk, and made hills and valleys in the places thereof; and the inhabitants thereof have I buried up in the depths of the earth, to hide their wickedness and abominations from before my face, that the blood of the prophets and the saints should not come up any more unto me against them.
And behold, that great city Jacobugath, which was inhabited by the people of king Jacob, have I caused to be burned with fire because of their sins and their wickedness, which was above all the wickedness of the whole earth, because of their secret murders and combinations; for it was they that did destroy the peace of my people and the government of the land; therefore I did cause them to be burned, to destroy them from before my face, that the blood of the prophets and the saints should not come up unto me any more against them.
And because they did cast them all out, that there were none righteous among them, I did send down fire and destroy them, that their wickedness and abominations might be hid from before my face, that the blood of the prophets and the saints whom I sent among them might not cry unto me from the ground against them.3 Nephi 9:5, 7-9, 11
So maybe the destruction to destroy the wicked started in 3 Nephi 2! But why couldn’t it all happen at the same time?
And it came to pass that it did last for the space of three days that there was no light seen; and there was great mourning and howling and weeping among all the people continually; yea, great were the groanings of the people, because of the darkness and the great destruction which had come upon them.
And in one place they were heard to cry, saying: O that we had repented before this great and terrible day, and then would our brethren have been spared, and they would not have been burned in that great city Zarahemla.
And in another place they were heard to cry and mourn, saying: O that we had repented before this great and terrible day, and had not killed and stoned the prophets, and cast them out; then would our mothers and our fair daughters, and our children have been spared, and not have been buried up in that great city Moronihah. And thus were the howlings of the people great and terrible.3 Nephi 8:23-25
I think that there was enough destruction in those three days. The righteous already were brought to breaking point; maybe they wouldn’t have been able to stand anything else.