How did Hearns pummel Duran so mercilessly

Duran v Hagler
Another conversation but since it’s mentioned, I always thought there was a case for Duran eeking it out.
Duran won most of those cagey rounds where only a few clean punches were landed. I agree with the scorecards that Duran was up before the last two rounds anyway. I’ll have to watch that one again.