If you believe in conspiracy theories, don't bother reading this.
I like pilotError's links as credible explanations. The following is my personal experience as how I'd make sense of this.
All NASA documents are recorded and assigned an archivist. Archivists are responsible for receiving, cataloguing, storing, preserving the artifacts. As a former archivist (not NASA), I know that inheriting video footage means spending time viewing, tagging, and deciding how to catalogue the video. Sometimes, cataloguing means just logging the date, broad subject, and amount of footage; frequently, cataloguing means just logging that you have received an artifact from a source on a certain date and that you'll get to it later. This can pile up, rather dramatically, especially if you have endless storage space. Remember the last scene in "Indiana Jones and the Lost Ark"? That could have been any of the institutional archives I've worked in. Couple that with the mobility of archivists, changing cataloguing and filing procedures, sloppy recording of what and where things are, and you have a researcher's nightmare.
Two years ago, I wanted to do do some cognitive research using some of the footage from various space exploration missions. I started by researching on-line. Some VHS/Beta tapes were indexed, and readily available to the public (and as of Jan. 2005, the video catalogue (NASA Video Catalog, January 2005, NASA/SP2005-7109/SUPPL15 - videocat.pdf - sorry, didn't think of keeping the URL) does list the Apollo 11 flight and moon landing). My research, though centered on seemingly more obscure catalogues, and I was looking for over 100, 000 feet of unlogged, undigitized videos. My search quickly degenerated into a labyrinth of where these various videotapes could be stored by NASA (Wash. D.C, California, Cleveland, Texas, and Florida, for starts). Getting access to the footage was intimidating (I couldn't even easily find out who was locally responsible for a section even with pretty impressive contacts, and I temporarily put this research aside). The bureaucracy is maddening.
I have no doubt that the footage will re-emerge, either as the original or digital copies. Maybe not in my lifetime, though.