Somewhere around 550 BC, when Nephi would have been approximately 60 years old, and about 30 years after his family left the Middle East, he writes these words:
Now I, Nephi, do speak somewhat concerning the words which I have written, which have been spoken by the mouth of Isaiah. For behold, Isaiah spake many things which were hard for many of my people to understand; for they know not concerning the manner of prophesying among the Jews.
2 For I, Nephi, have not taught them many things concerning the manner of the Jews; for their works were works of darkness, and their doings were doings of abominations.
Book of Mormon, 2nd Nephi chapter 25Nephi says in the above passage that many of his people do not understand "the manner of prophesying among the Jews" or "the manner of the Jews".
Nephi himself was very familiar with the prophesying and manner of the Jews because he had lived among the Jews. He was a Jew. It had only been about 30 years since Lehi, Nephi, and their small group had left the area of Jerusalem, and so at most only two additional generations of Lehi's clan could have been born by then. With perhaps 20 or 30 people in the original party that left the land of Jerusalem with Lehi, there could not have even been "many of [Nephi's] people" by the time of Nephi's writing, even if they were extremely prolific in the art of procreation. Unless they had encountered an indigenous population upon their arrival and integrated with them in some fashion.
That doesn't even take into account that most of Lehi's original clan broke off (being called the Lamanites) and became enemies of Nephi's sub-group. As explained in 2 Nephi chapter 5, the only people from their group that went with Nephi "were those who believed in the warnings and revelations of God". (2 Nephi 5:6)
Furthermore, by that time, a significant number of Nephi's group would have had first-hand experience and memories of the areas around Jerusalem, so they would have known about the "prophesying" and "manner of the Jews". So, it seems to me that the only possibility regarding the "many of [Nephi's] people" who did not know about the customs of the Jews would have been the native populations who, upon Nephi's engagement with them, became convinced and "hearkened to [his] words" and "believed in the warnings and revelations of God". (2 Nephi 5:6)
Why did these many people not know about the Jews? The quote above from 2 Nephi 25 tells us that that was because the Jews practiced "works of darkness" that Nephi didn't want them to know about.
Over a short amount of time, then, the memory of what it was like to live in Jerusalem would have died out among the Nephites (although it seems that such ideas may not have died out among the Lamanites, the enemy tribe consisting of some of Nephi's brothers and their followers, whose posterity perpetuated through the centuries the main source of anger that Laman and Lemuel were "the elder brethren" of Nephi, and that it was their right "to rule over this people." (2 Nephi 5:3)
With little memory or sharing of customs, it is very unlikely that those Jewish customs would have prevailed throughout Nephite history. What sort of customs would they have had if the Jewish ones weren't perpetuated? If you were to search an ancient Nephite archeological site, would you expect to find Jewish architecture? Jewish weapons? Jewish clothing? Jewish money? Jewish trinkets? Probably not. What you'd expect to find are artifacts that matched the culture of the ancient Americans who already lived here when Lehi, Nephi, and their extended family arrived.
Customs that, because "many of [the] people" never knew about them, could not have been Jewish.