what shows that the delegate is a good arbiter

The Delegate moderating the debate chooses the fruit farmers because it is more logical for the person who can put the land to better use to get it. In my opinion, his logic doesn't make his decision right, but the fact that he is able to convince everyone that "owning" the property isn't as important as what you're using it for makes him an excellent arbiter. He sold his point, made his decision, and ultimately, he got what he wanted.