What's the difference between "the main reason" and "the chief reason"? I've heard both, but have no idea where exactly these two differ.
Why stop with two? You can also speak of the prime reason, the central reason, the primary reason, the principle reason, the foremost reason, the predominant reason and the pre-eminent reason. Like main and chief, they have much the same meaning, but are likely to be found in different contexts. Only a corpus search can reveal what those contexts might be.