What makes an anime/manga popular? How does it get into the mainstream?

With so many different anime and manga available in the world, there are bound to be many that grow in popularity much more than others. For instance, series like Demon Slayer/Kimetsu no Yaiba absolutely blew up in popularity in late 2019. Other series like One Piece and Naruto have stayed relevant ever since they began in the late 1990s, and it seems just about everyone knows what Attack on Titan is even if they never watched/read anime/manga. But what is it that makes these series so popular? The characters, themes, accessibility, plot, or something else completely?