Although Isaiah has rightfully earned a place as the near pinnacle of biblical poetry, it is also possible that Nephi’s emphasis on Isaiah came from his scribal training where Isaiah would have been used as an important teaching text. Combining a familiarity with the obvious parallels Nephi would have seen between the Assyrian invasion and the Babylonian invasion that he had lived through, Isaiah was also very relevant. As a people separated from their land, Isaiah’s themes of the return were significant.
Nevertheless, the people to whom Nephi wrote did not have that background. While modern readers sympathize, we often don’t realize the implications of Nephi’s statement. It is certainly possible that his people didn’t understand Isaiah because he had not taught his children, but it seems strange that Nephi’s and Jacob’s children might not understand it well, since their fathers did. It is more likely that the ones who did not understand were the indigenous populations that formed a large part of the Nephite culture at that time.
These new Nephites would be taught the important aspects of the Hebrew religion, but not the sins that had led to the Assyrian and Babylonian destructions. The very fact that Nephi used Isaiah tells us that the problem wasn’t Isaiah, but rather the sins of the people that Isaiah declared were the reason for the Assyrian invasion.
Those collective cultural sins were the part of the Jewish teachings that were not passed on. Faithfulness to the gospel was a good thing, and that was certainly taught.