Pokemon's popularity lies in a lot of things. In the beginning, I s'pose it really is comparable t' CoD4: the first generation are fantastic GB games, period. The same could be said for Gen 2, not that it was revolutionary as Gen 1 was since they're the same game in essence.
Combine the greatness of Retromons with brilliant marketing; ohai anime and manga; and Pokemon became a youngin' phenomenon on a scale overbearing CoD's popularity by a huge margin. Despite the fact that for two generations (3 and 4) Game Freak didn't do much t' keep old blood in the game (IMO), there's always new players to bring into the market. Popularity begets popularity. Even in the dark times of Gen 3, Pokemon's fanbase has been growin' and growin'. Just like how CoD continues to get more undeserved fame by the media even when you've got shit like MW2 staining that goodness CoD once had.
Also, just for the record, unlike CoD Pokemon had some legitimately great games developed long past it's prime years. Mystery Dungeon and BW1 are good examples.