One of the most fundamental principles of archiving is human readability. Prints and negatives (digital or analogue) are human readable and don't require migration like data (and even with computer data, there is a distinction between human readable document formats like ASCII or RTF that could be printed out and read straightforwardly as opposed to MSWord DOC files, which need software to be interpretable). Of course prints and negatives benefit from optimal storage conditions, but under less than optimal conditions, if a human readable object is found a hundred, five hundred, three thousand, or ten thousand years after it is made, it is possible to understand on some level what it is without additional technology. This is not true of machine readable data.
Libraries and museums digitize collections not to create an archival copy, but to disseminate the material and to create a reference in the event that the artifacts themselves are damaged and need to be restored at some point, just as they have done with microfilm, microfiche, 35mm slides, and large format negatives and transparencies. The archival object is always the original artifact itself.