Why do you think King Lamoni was blessed with a vision of the premortal Christ? Partly because of his faith and because of the righteous and devoted relationship between this king and his wife. The first thing Lamoni does after he arises was to reach out to his wife and say, “Blessed be the name of God, and blessed art thou” (19:12).
In addition, this is an extraordinary event. How many missionary converts have an experience like this after the first discussion? Not very many. Visions like this come at certain times in history at important moments. For the same reason, why did Lehi see what he saw? The Lord needed to call Lehi to do something very unusual and to start a whole migration—so that a whole nation could be faithful. Likewise here, the Lord was trying to raise up a righteous people. Don’t you think that the Lord must have known that there was a great opportunity here with king Lamoni? From Lamoni’s conversion came the people of Ammon along with other converts in the lands of Ishmael and Nephi. The results of the work done by the four sons of Mosiah for fourteen years also prepared the way to a host of converts, including Samuel the Lamanite, who were brought to a testimony of the truth of the Gospel by the missionary work of Nephi and Lehi, the sons of Helaman (in Helaman 5) about 55 years later.