I couldn't answer that one. 是什麼意思

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It depends on the specific context. In this case, it sounds like she is saying (in a nicer way) that there are too many factors to list, or perhaps the reasons are vague or maybe she left the show because of a dispute and she is nicely dodging the question. If she said, "I can't answer that one." it would carry the nuance that she is refusing to answer the question. By phrasing it this way, she is softening the tone, and the nuance is "I don't know."