That terminal values are like axioms, not like theorems. That is, they're the things without which you cannot actually ask the question, "Is this true?"

You can say or write the words "Is", "this", and "true" without having axioms related to that question somewhere in your mind, of course, but you can't mean anything coherent by the sentence. Someone who asks, "Why terminal value A rather than terminal value B?" and expects (or gives) an answer other than "Because of terminal value A, obviously!"* is confused.

*That's assuming that A really is a terminal value of the person's moral system. It could be an instrumental value; people have been known to hold false beliefs about their own minds.