This is the assumption of Creators that their audience is too incompetent to truly appreciate sophisticated entertainment. This belief is a root cause of Executive Meddling (but not the only cause), especially among shows intended for children. Kids can legitimately be said to be less knowledgeable than adults, though we all probably know a depressingly large number of exceptions.
Of course, there is a "moron" demographic out there, and it affects storytelling by the "80-20" rule—in this case the market-research shows 80% of money spent on television-advertised products comes from the lowest 20% in terms of education and intelligence, so show-content is naturally geared towards profits (without profits, they can't produce more shows).
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While the opinion is broadly expressed as "moronic audiences", a large number of similar complaints also count. The creator may view their audience to be ignorant, hate change, need to be instantly satisfied, or have the attention span of a goldfish.