Why does Hazel not cry for the death of her son?

Because her intelligence keeps her from focusing on the event

In "Harrison Bergeron," Hazel Bergeron has been described as a woman of average intelligence and short attention span. She does not remember what is going on around her for too long. She is unable to maintain focus on a single idea for too long. She does not have the ability to think for long or deeply about something to understand what is going on around her. She only feels momentarily sad and distressed about the situation but when questioned by her husband she replies that it is only due to something sad that she saw on television.