Why do Jains believe that some early teachings of the Tirthankaras have been lost?

In Jain tradition, the original, unadulterated teachings of the Tirthankaras were said to be contained in 14 ancient texts known as the Purvas (or "prior" texts), which are now considered lost.1

Jain belief holds that the transmission of these sacred teachings has been subject to loss and corruption over time. While both the Shvetambara and Digambara sects agree that the teachings have suffered from this process, they disagree on the extent of the loss, which has resulted in differences between their respective canonical texts.1

Historically, the Jain canonical scriptures were transmitted orally and underwent various revisions and additions across several councils, such as those at Patna, Mathura, and Valabhi. For example, the Shvetambara canon originally included 12 Angas ("Parts"), but the 12th text, the Drishtivada, was lost. It is a fundamental tenet of Jainism that a time will eventually come when all teachings of the Tirthankaras will be completely lost, leading to the disappearance of the religion from the earth until it reappears in the next time cycle (kalpa).1