It's because with the creation of pokémon there came more than just a story of a few heroes and all of the universe confining itself to their endeavours. They created a universe, where stories can unfold beyond just a focus plotline. There's place for everyone to realise their own stories within its framework, and imagine the unsung parts of its other inhabitants inside the franchise. It is succesful because of its open-endedness, never really reaching a finish line where the story ends and the universe ends, because everything that the heroes needed to do was done, and nothing else matters. Such a universe offers lots of creative freedom to its fans, and spawns a community that not only observes and loves the franchise, but can participate, be active, roleplay, draw, imagine new regions, new people, new stories, new pokémon, etcetera. It's this kind of formula that allows some other worlds to attain similar success: Minecraft, The Elder Scrolls, Digimon, Harry Potter... just take the main story, the main heroes, and the main locations right out of the picture, and it still stands.