Why does the Book of Mormon have the exact wording of the King James Bible in so many places?
As noted in Chapter 2, the Book of Mormon was undoubtedly translated into the language of Joseph Smith. In fact the Lord said that revelations are given to His servants in the “manner of their language” (D&C 1:24; emphasis added). Not only does this explain the non-standard grammar but it also helps us understand why the Book of Mormon is written in the King James English style. Joseph’s language, according to his understanding of what scripture should look like, would have been the King James English. Many modern translations of the Dead Sea Scrolls (discovered fewer than seventy years ago) have also been translated into the King James English. This does not mean that the Dead Sea Community spoke King James English, but rather that the translators used the King James English as a vehicle to convey the meaning and tone of these texts.
Michael R. Ash, Shaken Faith Syndrome: Strengthening One’s Testimony in the Face of Criticism and Doubt