I'm trying to explain to students the difference between colon and dash, but I'm having trouble explaining why dash is better than colon in the following sentence:
In 2016, Leonardo DiCaprio finally won an Oscar: on the sixth time he was nominated.
In 2016, Leonardo DiCaprio finally won an Oscar--on the sixth time he was nominated.
So far I've been telling students that both define, explain, or add details, but a dash adds a dramatic effect like a drum roll. However, in this case, I believe what makes the second sentence better is not the drum roll. What makes a dash better in this case, and when is a colon better than dash after "Leonardo DiCaprio finally won an Oscar"?