Rareness is relative--there weren't a lot of books, but there weren't a lot of people who could afford or read them, either. So an old book was just an old book

Rareness is relative--there weren't a lot of books, but there weren't a lot of people who could afford or read them, either. So an old book was just an old book. Printing was increasing the number and affordability of books, so saving an old manuscript may not have seemed a high priority. Possibly the rest of it was already lost/damaged.