We used to have archaeological records of scripts that date back to the Ancient Ages. They were written and were preserved by initial conditions or fortunate conditions that occur later after these scripts were written. Even in the Modern Age, after the advent of radio and telegraphs, cables can still be preserved and archived in paper, where they could be read again later for historians to write histories about that era.
But in this Digital Age, information moves as bits on hard-disks or electromagnetic storage, which are relatively more fragile than data stored in, for example, papyrus scripts or hieroglyphs written on walls that date back to 4,000 BCE. Imagine the last time your computer hanged incessantly and the only way to revive it back to life was only to wipe out the data and re-install the operating system. Now assume that you're going to be a future great-man, or a leader; but if you are going to be a future leader, how would historians write this part of your life that evaporated with the rest of your old operating system? Would they just evaporate like that or are there systems in place to preserve these (if not colliding with serious security and privacy reasons) artifacts?
The same goes with e-mails. I'm currently reading a biography of Mao Zedong, in which the author wrote, in detail, reproductions of Mao's cables to Stalin and other int'l parties. With communication gradually moving to e-mail (which in turn are stored in a hard-disk somewhere else in the globe), how would these communications be opened up later for history writing?
To close this, I would say that there's a problem too with file formats and file systems, which could be obsolete probably even five years from now (in twenty years, .doc files would be as obsolete as WordPerfect files you used to open on a Tandy 2000 system back in the 1983's.)