>>46425141Nostalgia: for obvious reasons. People grew up with it and it inevitably became part of their identities, influencing their social circles, hobbies and even personalities.
Variety: diverse designs with wide appeal. There's at least one pokemon for every taste imaginable. There are also several ways of appreciating the franchise, be it games, anime, fanart, whatever you want.
Consistency: the world the games take place and the gameplay mechanics have stayed more or less the same since the beginning. If you knew Pokemon 15 years ago you'd still be familiar with most of it if you picked it up now. Same applies to other parts of the franchise to a lesser degree.
Organization: believe it or not, but the fact that every Pokemon has a dex number and that generations come in similar year intervals helps a lot. It gives a sense of progression over the years and makes it easier for newcomers, as well as returnees, to get familiar with the creatures, the games, the anime, etc., which brings us to the next point...
Accessibility: being a franchise aimed at children, the basic concepts are super easy to understand. At the same time, the ambiguity of the pokemon world opens room for speculation for those who want to understand it more deeply.
Pervasiveness: this is both a result of its popularity and one of the reasons it is so big. The franchise tried to spread to every possible media it could reach since its early days, and managed to establish a presence there. There's basically no one in the modernized world today who hasn't seen or heard of Pikachu, because TPC have managed to put the yellow rat literally everywhere on this planet.
There are probably other reasons but I can't remember them now.
