1. Changes in market demand: Over time, market demand and consumer interest may change. This series of books may no longer meet the current market demand, resulting in a decline in sales, so they are no longer sold.
2. The content is out of date or out of touch with the times: the content of this series of books may be out of date or out of touch with the current times, which cannot meet the needs of readers. In this case, publishers can choose not to sell these books, so as to avoid overstock and unnecessary losses.