Laura Miller explores the reasons:
What kills a genre isn’t always clear. Supposedly, the readership for the western turned to urban crime fiction sometime in the 1970s. Why? Were they simply tired of cowboys and gunslingers, or had the myth of the Old West been too thoroughly undermined by counterculture critics and Native American activists? Other genres, like a certain flavor of softcore fictional titillation epitomized by the stewardess "memoir" "Coffee, Tea or Me?" — naughty, but not quite explicit enough to qualify as "adult books" — were made superfluous by the increased availability of straightforward porn.
Meanwhile, still other genres become so successful they metastasize, splitting into multiple subgenres.
The cozy mystery, perfected by Agatha Christie with her Miss Marple stories, has spawned a dizzying assortment of variations based on homey hobbies. There are baking mysteries, gardening mysteries and knitting mysteries. Paranormal romance — a hugely popular fusion of fantasy/horror, romance and (often) detective fiction — has cycled through creatures ranging from the predominant vampires to werewolves, fairies, demons, djinn and even dragons as the heroine’s love interest.
Jennifer Coburn complements Miller's focus on the decline of chick lit.