First, ancient science and technology were underdeveloped, and the number of books printed was limited, especially some books that were not valued by the mainstream, because Confucianism was the main idea in ancient times, and some books in other aspects might not be valued and few people paid attention to them. The amount of printing or copying is very small, and it only flows in a small range. If there is a war or something, it may disappear.
Second, some ancient rulers deliberately destroyed or tampered with some ancient books in order to rule, such as Qin Shihuang's burning books to bury Confucianism, and the changes made to the content when the Qing Dynasty edited the Siku Quanshu.