Jihyo is their leader and the leader is the one responsible for the group. So as a leader, she should watch over her teammates. She should always check if they're fine or not.
But she failed at that. It was a simple job, and yet, she failed at it.
She failed at being a great leader. She's the reason why that happened to her.
A day won't pass without her blaming herself for what happened to her. A day won't pass without her crying for her.