The ruling class in the Middle Ages was not very literate. If you spent a lot of time writing, you were probably a monk, lawyer, merchant or something equivalent to what we would call a manager. Monks might not have needed to have much contact with the illiterate lower orders, but the others would have done. Hence we get things like the trial records of Montaillou or the miller researched by Carlo Ginzburg. And the clerical class could look back to precedents like the historians of classical antiquity, who made a point of interviewing any eyewitness they could find. The literate stratum of society was well positioned to package the expressions of the illiterate and pass them on.