Whenever I hear people recite speeches, letters, and historical accounts from Americans who lived through the Civil War, I'm struck by the exceptional quality of their writing. Compared to modern writing I read every day, those words sound like poetry to me. I don't notice this from earlier writing (though I haven't been quite so exposed to that) or later writing.
I have three theories, any or all of which may be bogus:
Selection bias means that bad writing no longer exists or is no longer put into modern works of history.
I associate the older style of writing with high quality writers of the era. Therefore, I'm being fooled into thinking that ordinary writers using words and phrases that were common in that time are of similar quality as exceptional writers who happen to use the same words. (I'm thinking of Dickens (not American obviously), Melville, Twain, and so on.)
People from the era really were better writers since they depended on the written word for day-to-day communication more than we do. Alternatively, teachers emphasized literature to a greater degree than we do today.
I suppose this might be better on English.SE, but is there any evidence from a historical perspective that my impression is correct?