I think at least part of the reason is that in the early-modern period books as objects still had a lot of the specialness, for lack of a better word, that they

I think at least part of the reason is that in the early-modern period books as objects still had a lot of the specialness, for lack of a better word, that they had when they were entirely handmade and extremely rare. With the spread of machine production books increasingly became mere vehicles of text, with the content more or less abstracted from the physical form.