Anything considered safe for children can potentially be licensed out for merchandise, which is nearly guaranteed to sell, making many shows 30-minute commercials, FCC regulations permitting. Once television animation became associated with children, the producers of animated shows began writing down to their presumed audience, which made animation outside the age ghetto less profitable than animation inside it. There are many sociological theories as to how and why this trope originated, but one of the most common theories is that it's a by-product of the rise of animation on television in the '50s and '60s with many adults uninterested in the low quality of many of these, and thus only kids being able to tolerate it, as well as television at the time being marketed as a way of keeping kids quiet and the rise of parental groups arguing for more government regulation on the content of these programs. Animation has the reputation of being a frivolous medium suitable primarily for children, under the age of 12.
