They were written in Latin until well into the modern era. Newton wrote his major work in Latin, for example.

Yes, I know. Why the confidence that this contradicts anything I said?

Edit: Re-reading I realise you were reading that as me thinking the books weren't written in Latin, whereas I was trying to get the parent poster to think a bit beyond "the books were written in Latin" to wonder why they might be written in Latin long, long after the fall of the Roman Empire.