I guess they were not writing stuff for "readers"? This might be a well-known theory of which I'm just unaware, but could it be that writing fulfilled a different role/need in those epochs?
Indeed, it's been my observation that the written register has almost entirely disappeared - there's only the spoken register in text form. But even that has declined in complexity: consider what Presidential addresses use to look like. Go compare Lincoln's Second Inaugural address with Obama's, for example.
> Indeed, it's been my observation that the written register has almost entirely disappeared
I think that's a lot of it. Then again, maybe people in the 19th century US spoke like that, too? I haven't looked into it. Surely there's a book about this....