Against creature heavy decks,
Duress is often a dead card or very limited in what it can remove.
You may even find nothing to remove when you play it, resulting in 1 card loss and 1 mana cost for a
peek at your opponent's hand.
With
Thoughtseize you can always hit the most problematic/best non-land card your opponent has in hand.
As long as the opponent's hand is not empty or not just containing land you always hit something.