The printing press certainly was a key element, but I think there has been too great a focus on the press as "an agent of change" with regard to book making. Large-scale book printing requires some kind of material to print on and that would also have to be produced en masse, and this material would have required some kind of resource which also would have to be produced. The material in question was paper and its production in paper mills North of the Alps started in the late 14th century and expanded from there until the onset of the Gutenberg press, after which it exploded with a stream of improvements making the process more efficient. The corresponding resource was linen whose production also expanded in the region North of the alps in the late 14th century, as the regional and inter-regional trade expanded. Hence, the printing press at the center of a larger process of commercial expansion and technical innovation that enabled the explosion of book production, in the first place. Eisenstein herself points out that the introduction of paper as a cheap mass product alone did not have an effect on the scale as the printing press, which is obviously true, but even though it was not a sufficient condition, it was a necessary one.
To some extent you can't really point to a single cause of anything, history is just the evolution of human societies within their ecosystems.
It is interesting to compare China and Europe. China had paper well before Europe did (Europe had papyrus but that was too expensive for ordinary use). Europe learned how to make cheap paper in the 13th century (and this process was learned from Baghdad, where it had been transmitted to from China). The Chinese also knew how to print by moveable type and again it took much longer for Europe to find out about the technology.
A common factor seems to be that Europe was open to accepting new ideas from far away. It readily adopted new processes from other cultures. I see little evidence of this in the case of China.
It seems that historical/cultural factors may have made the difference. Jesus was born in the 1st Century, near Europe, unknown to China. Christianity spread in Europe, and its message about the fate of human beings in the after life engendered an earnest desire for religious texts. So i think that in Europe there was, because of Christianity, a real economic demand for books; this demand could have been the driving force to invest in the technology to make books.
The search for ancient religious books resulted in the incidental disovery of non-Christian works which were almost discarded as "humanities". The ability to produce books in quantity conveyed these ancient texts, as well as religious ones, to the reading public.