This answer varies from person to person, but for me, it's all about nostalgia. But, of course, it also doesn't hurt that I find the little monsters adorable! Pokémon does a great job at staying young enough to pull in new children but growing with their fanbase to keep the  life-longers happy. They do this by constantly introducing new elements such as shiny Pokémon to add a difficulty level to both the games and collectibles!