While I fully agree about competition of new epitomes of earlier works driving original, more detailed works to extinction (that's how we lost many historical sources and lexicographical works), I would add that the driving force of this competition was economical. Medieval books were a luxury item costing to an order of magnitude as much as an average annual income of a worker, so most secular books that were copied had practical applications, and among those works we could consider arts and sciences most popular were those from the school cirriculum. Euclid had got included in it, but those older authors had not. And BTW, even modern authors don't include much historical details in school textbooks on math.
Regarding the lost works, when stored properly and protected from moisture, papyrus is about as strong as parchment, and in Ancient Egypt 250-year-old unneeded papyri were sometimes considered preserved enough to use again. However, unlike parchment it becomes very fragile if low and high humidity alternate, which is typical for European climate. Fungi (like molds) and insects (like ants) may also destroy the papyrus for the sake of calories. As for the parchment, even though it's more easy to store in European climate, being untanned skin, it's no less nutritiuos to living beings from the size of mice all the way down to bacteria.
Ancient Alexandria in particular is now below sea level due to a very powerful earthquake, but the library dwindled due to lack of financing a century before that.
Also as noted by historians, during the early period of archeology a great deal of excavated papyri were destroyed due to multiple factors.