Here's why:
* Ponyboy is conflicted: He knows that Johnny killed Bob in self-defense, but he also feels guilt and responsibility for the situation.
* Randy's perspective: Randy is Bob's best friend and deeply grieving. Ponyboy doesn't want to add to Randy's pain by revealing the truth, especially since it wouldn't change anything.
* Thematic importance: The book explores themes of social division, prejudice, and the complexities of justice. The ambiguity about who killed Bob reflects these themes, forcing the reader to consider the nuances of the situation.
Instead of telling Randy directly, Ponyboy leaves it up to him to figure out the truth. This creates a powerful moment in the story where Randy, despite his anger and grief, shows understanding and compassion for Ponyboy and Johnny.