Newspapers and broadsheets are meant to be consumed on a daily basis, by people of all income levels. This presumes near-universal literacy. Professional authors, who support themselves by book sales, rather than by patronage. Authors can't make money unless they sell lots of books, or fewer books at a high price. Lots of books implies lots of buyers, and therefore lots of readers.
Mass communication is in printed form.
We post messages rather than hire a crier, run personal ads rather than see a matchmaker, and converse electronically. Our laws are published, not read. Clearly, writing is one of the communication media of choice, not obligation. Although it isn't necessary per se, we make shopping lists and keep day-books. Prevalence of schools.