The Mulekites spoke Hebrew when they arrived in the Americas. Without a written record, their language naturally became corrupted. Imagine that the time Mosiah found them was 200 BC. If such was the case, the Mulekites would have been on their own for about 290 years. Imagine how much the English language has changed in the last 290 years. If spoken English has changed that much with the benefit of many written records and the stabilizing force of the Bible, you can only imagine how quickly the Mulekite language was transformed.
Now we begin to more fully understand the wisdom of the Lord in sending Nephi and his brothers back to Jerusalem to get the brass plates of Laban. Without this record, the Nephite language would have similarly been corrupted. Even with the record, the language was naturally modified, as Moroni states, the Hebrew hath been altered by us also (Mormon 9:33).
The most significant effect of not bringing any records with them was that the Mulekites denied the being of their Creator. The story of the creation is pivotal to our understanding of God and the plan of salvation. The Nephites had this record in the brass plates, but the Mulekites had lost touch with their God because they had no record to teach them of the creation and His dealings with the house of Israel. Imagine how many lives would have been blessed among the early Mulekites if they had been wise enough to preserve their religious tradition.