The Nephite apostasy is particularly disastrous because of the great blessings they had previously received. This is the enduring difference between the majority of the Lamanites and the Nephites. The Lamanites who had not known the gospel never did willfully rebel and therefore do not come under the same condemnation as the Nephites. For Mormon, this greater knowledge is the reason that this rebellion will lead to their destruction. The Lamanites, however, would be preserved.
Mormon shapes his history to present an earthly parallel to heavenly salvation and destruction. Note how Abinadi contrasts the eternal fates of those who have received the gospel and those who have not:
But behold, and fear, and tremble before God, for ye ought to tremble; for the Lord redeemeth none such that rebel against him and die in their sins; yea, even all those that have perished in their sins ever since the world began, that have wilfully rebelled against God, that have known the commandments of God, and would not keep them; these are they that have no part in the first resurrection.
Therefore ought ye not to tremble? For salvation cometh to none such; for the Lord hath redeemed none such; yea, neither can the Lord redeem such; for he cannot deny himself; for he cannot deny justice when it has its claim. (Mosiah 15:26–27)
Whether Mormon’s use of Abinadi’s exact phrase “willfully rebelled” is intentional or coincidental, the conceptual parallel is clear. The fate of willful rebellion is destruction. Now that the Nephites are in such a state, they are ripe for destruction, and Mormon is fully aware that, in historical terms, their destruction is near at hand.