I must confess that I have never understood this part of the scandal. The National Archives would be one place I would expect people in charge to be fastidious about guarding and filing and keeping backup copies if the originals are to be accessible to people.
The originals were not accessible to anybody without extremely high security clearance. Increasing the number of copies of something in existence tends to increase the likelihood that one of them will leak. Of course, if one lets Sandy Burglar have unfettered access to the documents all bets are off.