Helaman writes to Moroni because Helaman has been active in the western/southern theater and Moroni is still occupied on the eastern/northern front.
Text: The letter, like Moroni’s to Ammoron, confirms some level of literacy among the Nephite elite—which is precisely where we would look for it. During the later Maya Classic period (for which there is textual evidence), scribes were apparently a specialized group, probably from elite families who were not in line for the throne. Helaman, a Nephite religious leader, and Moroni, a captain, are literate. Perhaps they are also members of important clans. It would be unusual for either of them to have attained their positions solely on merit rather than the assumptions of their status in a kin group.
Culture: Almost certainly this letter would have been written on some type of paper. (See commentary accompanying Alma 30:51.) Its length and the need to carry it to Moroni both counterindicate metal plates, which must have been reserved for royal, official, and permanent records. The need for haste in a military situation would also contraindicate the use of metal. However, it is probable that, in order for this letter to have been preserved over the four hundred plus years separating the writing from Mormon’s redaction, it had been inscribed on the plates of Nephi as part of the official record.