No one at NASA seems to know, but my guess is that at some point the space agency became aware of the need for longer lasting documents, particularly books and reports. This may not have been the case in the early days of the space program, when acid-free stock was rare and expensive. A varnish is a clear coating that helps prevent fingerprints. Then there is fading. Depending on how much ultraviolet light the litho is exposed to, the vegetable-based inks used in lithography can be severely damaged.
Fluorescent lights are the worst, giving out large amounts of color-sapping ultraviolet light.
The shorter blue and ultraviolet wavelengths bounce around more readily then longer, redder wavelengths. Flesh tones and moonscapes grow colder. The difference is subtle at first, and you may not even notice unless a full-color version is seen alongside. You are most likely familiar with the analog photographic process. Far from it.