Why do we say "In the country of the blind, the one-eyed man is king"?

Meaning:

Among incompetents, even mediocrity passes for brilliance

Background:

The first known reference to this proverb is in Why Come Ye Nat to Courte?, a poem by  Englishman John Skelton, published around 1522. The poem is one of a number in which he takes Cardinal Wolsey to task, with pithy lines such as these:

He is so ambicyous
So shamles, and so vicious
And so supersticyous
And so moche oblivious…

Some years before, Skelton had been appointed tutor to the future Henry VIII and was on good terms with Thomas Wolsey, but he ended up in prison later in his life, most likely having offended the, by then, very powerful Cardinal Wolsey. Not long after, Skelton retired from court and became rector of Diss, a town in the East of England, well away from the intrigue of the Tudor court.

Emasmus, who was acquainted with Skelton and his poetry, expressed a similar thought in "Adagia" a few years later: In regione caecorum rex est luscus (in the country of the blind, the one-eyed man is king).

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