The Metroid Prime games have been about merging shooting elements and adventure game elements in an attempt to create a game which is unique; unlike most FPS the enemies each have distinct weak points which need to be exploited, there is a focus on exploration, puzzles and boss battles. Hypothetically speaking, the games have always had the potential to sell huge quantities.
Both MP1 and MP2 were heavily hurt by the low popularity of the Gamecube but Metroid Prime 2 was released at the worst possible time. Q4 2002 saw the release of Metroid Prime 2 (Gamecube), Halo 2 (XBox), Half Life 2 (PC), Grand Theft Auto: San Andreas (PS2), World of Warcraft (PC), and the Nintendo DS. To a certain extent I think this is why Nintendo is targeting an August release; it will give Metroid Prime 3 a month (or possibly longer) before the other companies release their major titles this year (GT4, Halo 3, Killzone 2, etc.).
