Literacy rates of higher socio-economic classes have always been greater than the lower classes. This is true today as well as in the Middle Ages. Though books are no longer status symbols
Medieval manuscripts are published online by many different entities, the chief of which belong to large groups of professionals working together: therefore we have national libraries, regional/national museums, university libraries, and religious institutions.
Each of these institutions has their own methods and motivations in producing these manuscripts, which affects exactly what they choose to put online. The Welsh National Library places its mythological texts online before it does the entire Chaucer manuscript they have, which would be of international interest, and the Metropolitan Museum of Art uses only brief sections of the book of hours of Jeanne D’Evreux to describe the wider area of manuscript illustration. Hopefully it will become clear just how the are related as you read further through these links.