Why do elite captains walk away instead of killing me?

In an attempt to get a captain promoted to warchief I tried to get myself killed by an elite captain (the last row before the warchiefs), as a comment to this answer suggests. However, when I was beaten, the captain simply walked away saying something about me not being worthy enough for him (if I remember correctly).

At first I thought I made it too easy by not fighting back at all, but I tried it with another elite captain and the first one a second time, hitting them a few times and reducing their health bar before getting killed. But again, both just walked away from the fight and "teleported" to another area. The elite captain I fought twice only said something like "how does it feel being beaten a second time?" and walked away.

When time passes and the changes are shown in Sauron's army, they also don't get a power increase for defeating me.

Why do they decide to let me live? Is this to prevent them from being promoted to warchief?

That particular enemy had the "Humiliator" strength.

The text of it says "Does not finish off weak opponents." Perhaps you have to do a certain amount of damage to them before they'll finish you off?

The ign wiki has a full list of Nemesis traits.