It would be relatively modern to keep a single volume in plain text as we commonly do today. Practitioners of earlier times would often keep their work in some kind of “code” and rituals were generally not written down whole cloth, both for safety (not getting inquisitated) and to ensure they didn’t fall into the hands of the unprepared. Instead rituals were memorized or parts were entrusted to various individuals within the working group.
It can be postulated that this is part of why the rituals that Gardner encountered in his initiatory group were so fragmented. After he went through the effort of shoring them up with bits and pieces taken from other ritual groups he had participated in, surely he wanted some way to ensure they would remain intact, and therefore instituted the tradition of the coven Book of Shadows.