She talked with the Companion about some of the reasons why the Harry Potter books have sold so well. They have to follow the formulas that have appealed to the target audience in the past.
Popular literature is directed at a limited audience.
A given work is not aimed at just anybody. A successful work is creative and fresh, but always within the limits of those formulas. Violating those formulas will disturb the audience in ways it does not want to be disturbed when it picks up its favorite kind of pop lit. There are some exceptions. Bridges of Madison County is an interesting example.