I work with government records on a state basis but it's basically the same premise as federal: When you have a volume of records that take up tens of thousands of cubic feet of space, like FBI records, at some point they need to be destroyed. I imagine they have a set retention period for records to be destroyed and unless they hold some type of major historic value (Kennedy assassination stuff) or are constantly being requested (guys like Gotti, Giancana, etc.) than they are probably seen as something expendable which can be destroyed.