Writing lends persistence and faithfulness to the original.
Oral traditions tend to be divergent and mutable.
Even such changes as the introduction of the printing press, and cheap paper, had profound impacts:
• Copies were now letter-for-letter identical.
• Reproduction costs fell by 2-6 orders of magnitude.
• Translation to the vernacular became viable. It was cheaper to bring the book to the reader (translation) than the reader to the book (learning Latin, Greek, French, German, Italian, etc.).
• Literacy grew -- from 5-10% in 1500 or so to 95%+ by 1900.
Of course, the number of published works grew, as did the literary canon, to the point it's a largely meaningless term (or has been subsumed by pop culture).