A hard drive is constantly in use and it's mechanical, mechanical things break down, the drive is also open to other kinds of failure as happened to me the other week, the drive I keep my current images on 'lost' it's file structure and there was nothing I could do to fix it but reformat.
A DVD is inert, once written you put it away safely till you need it and even when it's in use it's in a non destructive manner. That said, nothing is perfect even DVDs are meant to degrade with time but as long as you buy decent ones and store them properly they'll last a lot longer than a hard drive will imo.