May I ask a naive question... why is Persona so popular?
Because it's good.
And "so popular" is pretty relative. Compared to what? It's still a relatively niche series that appeals mostly to enthusiasts. The only Persona game that has sold nearly a million copies worldwide is probably Persona 4 Golden (maybe Revelations: Persona). Persona 5 is the game that is looking to truly popularize it way more.