In the early 1980s, to encourage aggressive sales, the U.S. government raised the taxes charge on goods left in warehouses at the end of each year. As applied to books, this meant that publishers could no longer afford to keep large numbers of titles on their backlists (the still-available books published in previous years). Books that had been in print for decades suddenly became unavailable. Now, only those titles that still sell well remain in print for very long.

The Pleasures of Children’s Literature, Nodelman and Reimer

(Add it to the list of other reasons you already are sure to know about.)