Hey TC I actually wondered about this myself and studies it out as best I could. First thing is that while 3rd Nephi and Matthew are close, they aren’t identical. The Lord’s prayer section has significant changes from Matthew to the later Nephite visit. But still why does it resemble KJV and not JST? The Book of Mormon was translated in 1829 while the Bible wasn’t treated a few years later and then it wasn’t gone through verse by verse from Genesis to Revelation. Think about how much revelation the prophet had from 1829 till say 1832 was he ready for the light and knowledge provided by the JST in 1829 or was that growth in the following years crucial. We learn line by line and precept by precept, those few years after translating the Book of Mormon to was key to the JST. Looking back now the restoration seems like seamless package of thought and doctrine, but remember it took all of Joseph’s adult life to do. Think about this quote from Brigham Young, “Should the Lord Almighty send an angel to re-write the Bible, it would in many places be very different from what it now is. And I will even venture to say that if the Book of Mormon were now to be re-written, in many instances it would materially differ from the present translation. According as people are willing to receive the things of God, so the heavens send forth their blessings.”
That’s why when you read the scriptures something ‘new’ pops out at you. You weren’t ready for that truth last time but this time you are. Also the exact wording used is not so important as the truth taught.

Part of the issue is that Emma, Oliver Cowdery, Martin Harris all claim he never referenced a book during translation, including 3rd Nephi. So your response is far worse than fairmormon. Take your own advice before posting and research a little.