Why can’t the explanation for the novel’s decline be both/and? Because they were socialized by a common training in writing workshops to adopt a common set of tastes and attitudes, and because these included a taste for liberal attitudinizing, American novelists lost all interest in morality and manners. Or because they inherited a metaphysical view of the universe as bereft of morality and manners, they were quick to adopt the substitute offered in graduate writing programs.
I think the worst thing for any artist is to belong to a fashionable coterie. It's the sure way to create period pieces.