A reader points out to the previous one that the Aztecs lived in present-day Mexico, which is in North America, not South America. Another:
Your e-mailer asserts that the civilizations of South America are the pre-Columbian peoples Mormons believe were visited by Christ after the Resurrection, and not anyone in the area of modern-day Missouri. But the Mormon church has always been vague about the location of the civilization mentioned in the Book of Mormon.
Remember, for instance, that the golden plates on which the Book of Mormon was allegedly written were left in a hill in western New York, and a lot of the book seems to link its authors to the Moundbuilder culture of the Midwestern United States.
There's a lot of stuff from Mormon apologists on the internet that tries to nail down the exact locations of the places and events mentioned in LDS scripture, but a lot of Mormon writing seems to assume a pan-continental civilization, that all the inhabitants of North and South America and their culture are the result of a single family of Hebrew refugees landing in the New World c. 600 BCE. So where exactly Jesus appeared to the Nephites, much less anything else in the Book of Mormon, is pretty much an open question.
As for the actual importance of Missouri in LDS cosmology, it's allegedly the location of the Garden of Eden, or at least the spot where Adam and Eve lived after the Fall. Joseph Smith told his followers in the 1830s that upper Missouri would be their promised land, and devout Mormons to this day believe that prophecy will be fulfilled in some end-times scenario. So believing Jesus himself might show up in Missouri, either in the past or at some point in the future, doesn't seem that far off base.
The Religious Studies Center of Brigham Young University delves into the mystery at length. The intro:
Conflicting views exist about when Jesus appeared to his New World disciples. Did he appear directly after his ascension to the Father? Some believe that his appearance followed the forty days with his disciples in Palestine, while others believe that an entire year had passed after the resurrection when he appeared in the Americas. Observations from the text suggest that he mercifully waited for the people to recover from the destruction that attended his crucifixion. Compelling details help us approach an answer to this puzzling question.