A cold joke: Why did Wei Yan shout "Who dares to kill me" three times?

This cold joke is like this:

After Zhuge Liang died, Wei Yan rebelled. Yang Yi had no choice but to open the bag of tips left by the prime minister, which told Wei Yan to shout three times in front of the battle formation, whoever dares to kill me, someone who will kill him will appear. Yang Yi followed the plan and told Wei Yan that if you say loudly three times who dares to kill me, I will surrender. Wei Yan didn't know what was going on and shouted three times triumphantly. A man behind him suddenly rushed up on his horse and shouted: "I dare to kill you." He raised his knife and cut him off the horse. This person is Ma Dai.

There are many generals in Shu. Why did Zhuge Liang single out Ma Dai, who was not the best in martial arts, to deal with Wei Yan?

Because Ma Dai, named Ding Lin, specializes in governing Wei Yan