When writing the history, the writer is sitting outside the time, in Olympian detachment, surveying what was said and done, with the knowledge of what overhyped fads will fall by the wayside, and what ignored actions will prove to be crucial. He hasn't got that for the present era; the writer is still meshed in the circumstances that lead to the hyping and the ignoring. Not to mention that he is very likely to be a partisan in the matter -- most who write histories of a thing are passionately attached to the thing itself. Which can also lead to a shocking change in tone in the last chapter too, as the calm recitation of facts gives way to the sound of axes grinding, even if the writer manages to make interesting observations.
The irony is that anyone who's done the history of things will have read, in his research, many, many, many writers making idiots of themselves because they do not realize they are enmeshed in their era, and yet this does not stop doing the same thing over again.