To stay in circulation, books need copies. A book which stops being copied will slowly disappear.
In the Middle Ages, the rate of disappearance of books copied by hand was extremely high. As the literate demand for books almost always outstripped the supply of manuscripts, books copied in insufficient numbers would quickly fall out of circulation. Manuscripts were also less safe back then than printed books are today.
To keep books breathing and relevant to an age, they had to be constantly duplicated. Uncoordinated scribes and copyists working all across Europe kept some books on the shelf and allowed others to vanish. Today, we know of only a fraction of the books produced in the medieval period. Even fewer have survived. A lot of known unknowns but many more unsuspected unknowns.
By the 14th century, there were two book markets running parallel to each other. One was the market for books in circulation. These were popular books whose established demand assured their survival. The other was the market for nearly-vanished books, which the first humanists, Petrarch, Collucio Salutati, Leonardo Bruni etc brought to everyone’s attention. The first market was being renewed through inertial duplication. The other was refreshed through what came to be known as Renaissance Humanism. Books which hadn’t been copied for centuries, and were gathering age-old dust, were discovered and recovered by classically-minded, often classically-obsessed scholars. Renewal came as a burst towards the end of the medieval period, the recovery of the classical past leading to deep innovations in the present. Renewal blasting into rebirth, out of which new possibilities suddenly emerged.