Whilst browsing the "friends of friends" page that I discovered this fascinating post from girliejones (who I'm not sure I know in person, I may do) about the loss of a letter writing culture. The question at the end being:
What's with us? Do we have no sense for the preservation of history?
I answered in the comments (so make with the clicky already), but wanted to keep my response here as well.
Short Answer: Yes.
Long Answer: This has been identified as a problem with history generally in that the 2nd half - 3rd quarter of the 20th century is (or may be) significantly less well documented than the first half.
In particular due to loss of information from legacy computer systems that have been de-commissioned. The information has either been lost completely or is unreadable due lack of appropriate devices.