Around 600 BC father Lehi saw a vision of Christ and his apostles. He was even told that Christ would come visit the Lehites in the New World 600 years from their departure for the new promised land.
And yet despite that very explicit prophecy for centuries the following prophets are clueless. Look at how out of touch Alma is.
Unless everyone else was as ignorant as Alma, why would anyone be impressed by Samuel the Lamanite's prophecy?
Why would this be? The answer could be as simple as this: Joseph Smith didn't know how he was going to bring about the advent of Jesus in the Americas during the earlier stages of writing The Book of Mormon, but by the time he finished up Moroni and went back to replace the missing 116 pages of manuscript with what became 1 Nephi through Words of Mormon he had the whole thing figured out and could include all the appropriate prophetic details.
Sound doubtful? Good. Doubt your heart out!