There's a blog offering the best explanation ever of how your book gets sold into bookstore chains, by the marketing manager at John Wiley & Sons. Here's a brief snippet:
But bookstores are businesses, not public conveniences. No store has
the responsibility to carry every book published -- although, to be
honest, that's a straw-man argument, since no one is asking for that.
(They're just wishing that their books, the books they like, and the books by their friends be spared the chopping block.) I market books for a living, so I can tell you an unpleasant truth: the order for any book, from any
account, starts at zero. The publisher's sales rep walks in the door
with tipsheets and covers, past sales figures and promotional plans, to
convince that bookseller's buyer to buy that book. In many categories
-- SFF is still one of them -- the chain buyers say "yes" the
overwhelming majority of the time. But not all the time. Sometimes, that buyer is not convinced, and the order stays at zero.