Well, as from a marketing POV, the main series Pokemon is just like another game that is reusing its own title and make new games, where this is the most common and yet the best way for a franchise to "elongate" its popularity life.
Look at the Final Fantasy series, The Tales of series, the Zelda series, the Super Robots Taisen series, the Gundam series, the Sonic series, the Rockman/Megaman series.... you named it.
Whatever game series, there is one common market strategy they use: Reuse the game title and make a new game. This then make the franchise live long. Almost of these games series had celebrated their 10th or 15th, or some the 20th anniversary, which tells how long they had existed, and not yet forgotten in the publicity. In order for the gamers public to always remember the old games, the game franchise will remake the old ones. This market strategy was also seen in some of the series listed above.
And yet, Pokemon's popularity is much higher than those listed above, its games are comparable to the Mario series. Why? Because not only remakes, it is also like the Mario series, it made many spin-offs that is not related to the main series. In the gaming marketing POV, this is also another way to enlarge a franchise's popularity.
But remember, Pokemon's popularity lies not only in the games, but also thanks to other medias like anime, TCG, manga, related merchandise, etc.
For the simple fact, Pokemon anime is now the longest anime ever in the history of Japanese anime, longer than any others like Doraemon, Dragonball, Bleach, One Piece, Naruto, etc, that was known that had more than hundreds of episodes, and some had aired for more than 3 years. Doesn't matter the differences in target audience and themes of the animes listed above, the simple fact that it is LONG already creates the "brainwashing" effects and increases its popularity amongs the public.
Many Pokemon fans should also knew already, there is the Pokemon Center - the OFFICIAL Pokemon related merchandise shop, built in many places in Japan. This marketing strategy was never been done before, but the Pokemon Company did it. I don't know did they economically earn anything, but they did earn further publicity and popularity.
But whatever the analyze, one fact is that the Pokemon Company put many effort into many things. The games, anime, movies, TCG, manga, related merchandise, advertising... etc. The combined amount is much more greater than any fiction franchises, which in return earned its humongous popularity of nowadays.
