Why could no-one else write a revelation for the Doctrine and Covenants like Joseph Smith?

Doctrine and Covenants 67:7-9 contains the following challenge:

Now, seek ye out of the Book of Commandments, even the least that is among them, and appoint him that is the most wise among you;
Or, if there be any among you that shall make one like unto it, then ye are justified in saying that ye do not know that they are true;
But if ye cannot make one like unto it, ye are under condemnation if ye do not bear record that they are true.

Of this revelation William E. Berrett recorded:

McLellin, perhaps under the urging of others, accepted the challenge. He retired from the conference and, in the solitude of his room, attempted to write that which might sound like a revelation from the Lord. On November 2nd he appeared again in the conference and with tears in his eyes begged the forgiveness of the Prophet, of his brethren, and of the Lord. He could not write a revelation. Try as he might, he could not write that which would sound as if it were a revelation from the Lord. Everyone who puts the matter to the test must come to the same conclusion.”
– William E. Berrett, Teachings of the Doctrine and Covenants [Salt Lake City: Deseret Book Co., 1956], 6.

Joseph Smith similarly wrote as follows:

After the foregoing was received (D&C 67), William E. M’Lellin, as the wisest man, in his own estimation, having more learning than sense, endeavored to write a commandment like unto one of the least of the Lord’s, but failed; it was an awful responsibility to write in the name of the Lord. The Elders and all present that witnessed this vain attempt of a man to imitate the language of Jesus Christ, renewed their faith in the fulness of the Gospel, and in the truth of the commandments and revelations which the Lord had given to the Church through my instrumentality; and the Elders signified a willingness to bear testimony of their truth to all the world.
– History of The Church of Jesus Christ of Latter-day Saints, 7 vols., introduction and notes by B. H. Roberts [Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 1932-1951], 1: 226.)

What was so special about Joseph Smith that the wisest of others could not copy what he did?

See:

Gospel Doctrine – Section 67

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