“Provide from among the entire people, God-fearing men of means, men of truth who despise money; and appoint them judges.”
~ Exodus 18:21

In one sentence, Moses’ father-in-law Jethro, defines the exact criteria needed for being a judge.

Why doesn’t he mention the obvious: total knowledge of the law?

Because there’re two levels of knowledge: information and application. While information can be learned, its application is dependent on good character. Although Moses could teach anyone the laws, only those of character could implement them.

Any old donkey can schlep books on its back, but only a mensch can take the wisdom to heart.