Mosiah 28:15

and they have been kept and preserved by the hand of the Lord, that he should discover to every creature which should possess the land, the iniquities and abominations of his people:

The Lord kept and saved them. He wanted to show everyone who lived in the land the sins and terrible things his people had done.

The Lord had preserved them so he could reveal to every people who lived in this land the sins and evils of those who came before.