Romance novels are sold in American supermarkets, drugstores and warehouse clubs far more than most categories. The channel came first, and much of the genre's form follows from it.
The rack reaches readers who are not browsing
A shopper in a grocery aisle did not come for a book, so the display has to work as an impulse purchase alongside magazines and candy.
Romance suits that position because readers of the genre buy frequently and are willing to try an unfamiliar author for a small amount of money.
Categories with occasional buyers do not perform in the same space, which is why the rack is dominated by a few genres rather than by books generally.
The format was built for the fixture
Mass-market paperbacks are sized to fit standard wire racks, printed cheaply, and priced low enough that a mistake costs the reader very little.
Cover design follows the same constraint, with the title and author readable in a rack that shows only the top portion of each book.
Series branding does heavy work there, since a familiar imprint logo tells a shopper what they are getting faster than any description could.
Distribution runs on a different system
These outlets are served by magazine wholesalers who stock, rotate and strip the racks themselves, rather than by bookstore accounts placing individual orders.
Titles arrive monthly and leave monthly on a schedule the store does not manage, which is why the selection changes as regularly as the magazines beside it.
Publishing to that rhythm requires a steady supply of new books, which shaped the monthly line publishing the genre is known for.
Reach extends well past bookstores
Many American towns have a supermarket and no bookstore, so for a large number of readers the rack has been the primary source of new fiction.
That geography built a national readership for romance that was never dependent on bookstore density or on review coverage.
It also kept the genre's audience partly invisible to the literary press, which reported on the channels it could see.
Digital changed the mix without removing it
Ebooks matched the qualities that made the rack work, offering low prices, instant supply and privacy, and they absorbed a large share of the genre's volume.
Physical racks contracted as a result, though they remain in many stores and still introduce readers who are not shopping online.
The habits the channel created, frequent buying and loyalty to a line rather than a single author, carried over intact into the digital market.