Abundant alternative partners make harsh trigger strategies rational.

Evidence

In a world in which there are many potential partners in cooperation for various purposes, the standard, relatively friendly tit-for-tat strategy might be too generous. In that strategy, I cooperate with you if you cooperated on our last round, and I defect if you defected on our last round. If we both start out by cooperating and we both follow the tit-for-tat strategy, we may readily establish a long-term cooperative relationship. Just because we have many potential partners available, however, we might often use the more draconian trigger strategy of dropping efforts with anyone who fails an initial attempt at cooperation. One failure at the outset of our potential relationship and you are out of my ambit. — Russell Hardin, Trust and Trustworthiness, 438