It's hard selling books in general: companies are merging, editors being laid off, bricks-and-mortar bookstores closing, large chain bookstores squeezing out independents, and online retailers squeezing out chain bookstores.
With a hardcover, you get two chances, a year apart, for the book to make an impact - often with a new cover featuring artfully crafted snippets of reviews, a new marketing campaign and maybe even a new publisher.