I'd say it was a combination of the growing economy and population, increasing urbanization, and the appearance and spread of universities (and other forms of schooling), which meant increased literacy and thus demand for books. There were some technological changes which made books cheaper and easier to produce, such as the shift from parchment to paper1 and the pecia system2, which probably contributed as well.
A good paper looking at this issue is Buringh and van Zanden (2009).
Up until the 13th Century, an important factor was the growth in the number of monasteries (the main centers of both book production and "consumption"), which followed the growth of the population and the economy. Beginning in the 12th and 13th Centuries, the combination of increasing urbanization and the spread of universities created a new source of demand for books.
As for the Black Death, Buringh and van Zanden have this to say:
The Black Death of 1348 and the resulting decline in population levels
had a complex effect on book production. In the short term, output
probably declined significantly ... However, after this temporary
decline, production rebounded significantly, and an even sharper
increase in output began, resulting in an almost tenfold increase in
the next hundred years.
1. Paper production in Europe seems to have started in Italy in the 13th Century and spread northward to other countries in the 14th Century.
2. The Pecia system essentially parallelized manuscript copying by chopping a book up into multiple parts, each of which was copied by a different scribe. The pecia system was driven by the rise of universities: it developed as a way of supplying books to the growing number of students.