I wonder if it's sun/water exposure, as I have dice from Reno (1951) that I picked up as a child that are in their original state of preservation... and I've seen dice at flea markets that were issued much later that fit your description.

I also wonder if composition has anything to do with the breakdown/damage.

A similar reaction is found on older (Monte-Carlo and Vichy) casino plaques that came over to the USA years ago. It could be those sacks of plaques were exposed to moisture, because other pieces of the same denomination (same series) that have been cared for look like they were just made yesterday.
Perhaps one or two of our dice collectors can explain this breakdown process.

JB