Why do modern manuscripts of the Avesta date from the 13th century or later?

Modern manuscripts of the Avesta date from the 13th century or later because the text was preserved through an oral tradition for centuries after the language ceased to be used as an everyday spoken tongue around 400 BCE.12

Although the canon was fixed between the 4th and 6th centuries CE—at which point the priestly tradition was recorded in a specialized Avestan script—the surviving manuscripts represent this later transmission. The reliance on oral preservation, combined with the historical challenges faced by Zoroastrianism following the Islamic conquest, explains why earlier physical copies are not extant. These later manuscripts remain vital as they reflect the recording of that ancient priestly tradition.123