Another important factor was that during the Middle Ages a large proportion of the population would have been illiterate and so pictures on a sign were more useful than words as a means of identifying a public house. For this reason there was often no reason to write the establishment's name on the sign and inns opened without a formal written name, the name being derived later from the illustration on the pub's sign.
Try to answer this question if possible (otherwise reply "unanswerable"): In what historical period was a large portion of the population illiterate?
the Middle Ages