The papers and emulsions that made up the bulk of photographic records of last century were sturdy enough things to start with but when people failed to take care of them they frequently did not survive the lives of their subjects.
The autochrome on the top of the post is an example -the original plate was made with multi-coloured grains of starch that were dyed dufferent colours and then exposed through a tri-colour filter pack to yield some of the first colours shots. Unfortunately starch is ideal for the growth of mould if there is a damp and warm atmosphere. And it is nearly impossible to remove it without losing the image.
I will say that the manufacturers of the materials did not help in the mid part of the century. The Kodachrome slides stayed very well, and we can all be grateful for the fastness of the colour of both the original Kodachrome A and the improved Kodachrome II. But the Kodacolor films and particularly the Kodacolor prints avaiable in the 50's contained dyes that have proved so fugitive as to defy even the best restorative programs on the Epson scanner or the computer programs.
Really, when you see the sorts of problems that occur for the dedicated historian or just the family picture collector, you really do have to thank your lucky stars that we are shooting digital these days. We just don't have these sort of problems.