Why? It's not like they continued with the "Secret of Mana" name for the rest of the series (okay, technically Dawn of Mana is the only other mainline game) . If they'd change the "Final Fantasy Adventure" / "Mystic Quest" name, they'd probably change it to "Adventure of Mana" like they did for the remake.
Not close enough to the Japanese title. Let's aim for maximum brand confusion with Final Fantasy Legend: Adventure of Mana.