We are asked this question frequently, usually by authors of books in a category we don't publish. The author′s follow-up comment is usually along the lines, “But this is a hot topic, you could sell lots of books!” The answer is simple.
Book retailing is still dominated by brick-and-mortar book stores, despite the dramatic growth of selling on the Internet. This means that the people who make the book buying decisions at the largest retail chains hold the key to bringing your book to market.
Retail chain buyers are specialists: the large chains have different buyers assigned to different categories of books. One buyer may specialize in the Romance category; another in Computer books; and a third in books for Children and Pre-Teens. These buyers, as with any employees in a large business, are judged by their performance: Do the books they buy for the chain sell well, or do they tend to sell badly?
As a publisher selling books to a retail chain buyer, you know that the buyer is looking at the history of your previous sales to her. If your previous sales have been good ones — the books achieved respectable in-store sales —she will be more inclined to listen carefully to your presentation of new titles, and to place decent-size orders for those new titles.
If you are trying to sell a book in a category you have not previously published, you are now selling to a buyer who has no previous history with your books. This is like selling door-to-door: the best you can do is point to the house down the street where you have sold before, and encourage the buyer to check with her neighbors who will say the different things you sold them in the past were good.
At best, you may get a very small, trial order — the buyer may place a few copies of the new book in a handful of stores, then let the corporate computers tell her (a few months later) if the book sells enough to warrant buying more copies.
The odds are against that reorder happening: the limited number of copies bought means each of the "trial" stores only received two or three copies at best and they quickly disappeared in the shelves, overwhelmed by the larger numbers of copies of other titles in the category.
This is why we, and most book publishers, are very cautious about stepping outside the categories in which we have built our business and our reputation. We know we can do a better job for an author whose book "fits" inside the arena in which we sell.
The Internet and online selling may come to dominate book selling in years to come. Traditional (brick and mortar) book chains are suffering in the current recession while online vendors are doing a little better.
New ways of producing and delivering books are emerging: e-books, print-on-demand kiosks, and other technology changes are changing how books are produced and delivered. But while they are interesting, they remain more "future" than "now."
For the present and immediate future, most publishers will be answering the "Why don't you publish?" question with, "Because I cannot sell it well enough to benefit either you or me."
Conclusion As an author looking to get your manuscript published, do yourself a favor: research who publishes books like the one you have worked so hard to write, and approach them with your manuscript. You will increase your odds of getting published considerably! Good luck!