I'm doing a master's in library science and archives, currently working a couple of internships processing archives. The answer is archives are big, complex, and time-consuming. One collection I work with is 131 cubic feet of records including papers, floppy disks, and photographic film. It's unprocessed, meaning the archivists haven't had a chance to arrange and describe it, which isn't a wonder considering the size of the collection — and that's only one of many in the backlog.

Even if a collection is processed, because of the volume of information in a given collection archivists typically don't typically describe every document. In a library you can catalogue every book, but that's not possible in an archive. And in an artist's papers, how can you know which document will be important to someone? How can you know what's artistically significant? The time it would take to research the background of every document (Was this script ever made? Is it interesting to anyone?) would be prohibitive.

Add into the mix that archives are chronically underfunded and archivists underpaid. This is coming from the unpaid intern who was asked to process a $33,000 acquisition last year. Fun times.