Because Moana is the only one in Motunui that believes in grandma that insisted the legends are true.
Grandma: But one day, the heart will be found. By someone who will journey beyond our reef, find Māui, deliver him across the great ocean to restore Te Fiti's heart and save us all.
Well, grandma clearly knows Moana has the potential judged by her personality (curious about sailing and the ocean as she was the only one got really interested by the story of legend among the other kids). Thus the ocean chose Moana as the chosen one to save the world therefore grandma decided to tell her the truth: who are we meant to be? The answer is voyagers. And Moana herself understands, once she restored Te Fiti's heart, the world would goes peace and their people could voyage again in which indicates she "delivered" their people back on the ocean to voyage where they're supposed to.
Disclaimer: These are my thoughts which it does make sense.