Here’s an answer to a great question by Devorah Fox courtesy of FSFnet…

The bookselling industry requires that publishers and authors categorize their books. That makes sense. Readers in search of something new to read usually know that they’re looking for a mystery, biography, cookbook, etc. They would not at all be pleased to find bookstores, be they brick-and-mortar or electronic, with no more organization than a rummage …

Source: What Makes It A Fantasy? By Author Devorah Fox – Fantasy & Sci-Fi Network