Because people who write memoirs, tend to adjust the facts to show themselves in a better light.
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Reading about how earlier historians wrote about the French Revolution. (APEX)
Without disagreement there can be no discussion. Not only in the field of history but in many of the scientific fields, discussion is necessary to move forward. Should all of the historieans in the world agree on a set history (one that is unverifiable) the field itself would halt. Sometimes wrong conclusions are made. If there are no historians willing to disagree with one another these conclusions can be perpetuated.
Because the language is so old
Because the number of extant sources is usually greater, and that makes the corroboration of details easier. There may be certain events or aspects of the modern period that have few sources, however, and in that case, reconstructing that part of modern history should prove difficult and challenging, if not impossible.
Besides the movie, it's true that there was a real "man in the iron mask" and is even mentioned in the Memoirs of Mme du Barry. He is disputed to be either the Duc de Vermandois, a twin brother of Louis XIV, or an elder brother of Louis XIV. However, it should be noted that the "Iron Mask" wasn't an iron mask at all. It was a simple mask of black velvet.