The humor in the story is a big factor. There are many comical situations, such as when the devils struggle to understand human technology or social norms. For example, their confusion about things like vending machines and modern currency is really funny.
The unique concept. It takes the idea of devils coming to the human world and having to do ordinary jobs, which is a fresh take on the fantasy genre. It combines elements of the supernatural with everyday life, like Maou working at MgRonald's.