Apparently, in those days, verse was considered to be by far the premier mode of written expression, and the best writing was reserved for it. The word "***" - the same in OE as in modern English - is very ordinary-looking (and ordinary-sounding) compared to the ornate and highly inflected words of OE, and it apparently was considered too dull-looking (and dull-sounding) for the precious medium of verse.
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