Determining why exactly Pokémon Go is so popular is a lot like trying to catch a wild Gyarados: illusive, frustrating, not likely to happen anytime soon. Nevertheless, a significant consideration is the game's fanbase and their willingness to create intertextuality in order to spread Pokémon Go to as wide an audience as possible, including non-gamers.
Initially Niantic encouraged a cross-media understanding of Pokémon Go by planting an Easter egg that references the original anime. After a series of failed experiments involving secondary attacks and gym colors, players realized that they could control the evolutions of their Eevees by naming them after the Eevee Brothers of Episode 40. This not only cashed in on twenty years of nostalgia, but also encouraged fans outside the franchise to start creating their own intertextuality.
A combination of luck and common character-types assisted them. Harry Potter, the most popular fandom in recent history, was drawn into the Pokémon Go world when fans realized that the team characteristics coincided with their Hogwarts houses: Ravenclaw/Mystic, Gryffindor/Valor, Hufflepuff/Instinct... and the lack of a team for Slytherin only encouraged the long-held joke that nobody cares for the house, or at the very least they're the villains (Team Rocket). Thus, anyone interested in Harry Potter now has a familiar avenue into the game. This work has continued across numerous fandoms, from one fan theorizing that Eleven (Stranger Things), Negasonic Teenage Warhead (Deadpool), and Furiosa (Mad Max) are all evolutions of each other, to another fan manipulating the ability to name your Pokémon in order to promote the ship of Magneto/Xavier from X-Men: "Do you want to transfer Magneto to the Professor?" Such crossovers catch the interest of non-gaming and non-Pokémon fans and they increase the pleasure of Pokémon Go itself by crafting new fictional possibilities. How awesome would it be if Eleven, Negasonic, and Furiosa were the same being, the ultimate combination of recent, feminist idols?
Of course, Pokémon Go's augmented reality allows players to treat the world as a fandom all its own, where trainers can 'crossover' by having their pets interact with the Pokémon, using it as a social justice tool, or even just an instrument of self improvement. Whether you're interested in the latest film or discovering historical landmarks, the intertextuality nurtured by both Niantic and their fans ensures that there's truly something for everyone when you're out to catch 'em all.