Memory was still the most useful way to pass down the Scriptures and the words of the Christian fathers. When the codex, the book, first became truly popular in the seventh to the eighth century A.D., it changed radically the way scholars studied. Now it was easy to page back and forth between passages in the same work. Now multiple works could be studied with relative ease. Books were less expensive to produce and more available.
Books lend themselves to detailed arguments, written with a beginning, a middle and an end.
They allowed for reference from one work to another. They lent a illusion of authorship, a fantasy that you could produce something truly unique. The movable type printing press made the book truly popular. Rather than producing works by hand, one copy at a time, books rolled off the press quickly, hundreds, then thousands, then millions at a printing. They were easier to edit, cheaper to produce and quickly available.