The Jews in the Old World did not know of Israelites on the American continent because God the Father commanded his Son, the Good Shepherd, that they should not know, owing to their obstinacy and unbelief. The Good Shepherd has many sheep. His sheep in ancient Israel, the Jewish nation, failed to understand what he taught them concerning other sheep (as recorded in John 10:16). Their unbelieving attitude kept them from receiving further instructions. When he mentioned other sheep who should hear his voice, some of them thought he was referring to the Gentiles (non-Jews in the Mediterranean world). But no, those Gentiles would not hear his voice directly, as did the Nephites; rather, he was manifested to them through the ministration of the Holy Ghost and the apostles. How that happened can be studied in the New Testament books of Acts through Revelation.
The pattern of the Savior teaching only certain groups in person was followed in the spirit world immediately after his death. He did not visit the wicked, ungodly, unrepentant, or rebellious who rejected the truth. Rather, he organized and commissioned a missionary force among the righteous to go forth among the unrighteous. This is certainly a manifestation of God’s great justice (D&C 138:20, 29–34).
The “other sheep” referred to in these verses and the first verses of chapter 16 are the Nephites, who were Israelites guided by the hand of God to the Western Hemisphere, as well as other groups of Israelites among the “lost tribes” of Israel. The revelations—ancient and modern—do not give us much information regarding the whereabouts of these “other sheep,” nor details about exactly who they are. We patiently await additional instructions on the subject. No doubt God has had various groups of righteous “sheep” on this planet and on many other worlds.
In fact, toward the end of the first day of his visit, the Savior commanded his New World audience to go to their homes and ponder the things he had said while he went to show himself “unto the lost tribes of Israel, for they are not lost unto the Father, for he knoweth whither he hath taken them” (3 Nephi 17:3–4).