Why? Because the essence of Ch'an is "sudden enlightenment."
Nobody in Chinese or Japanese Zen history seems to have believed that any of what a non-enlightened person says about Zen could ever be of the least interest or importance, any more than you or I would trouble ourselves over the idea of our names coming up in the gossip of inmates in a mental hospital.
An unenlightened person is, by definition, merely a drone speaking or writing from received ideas, which are delusions. To think that such a person could meaningfully critique or an interpret the words or doings of an enlightened Master is a glaring contradiction in terms.
One reads a piece of "Zen" writing -- whether a poem or a Dharma speech or any other fragment of discourse -- always and only because the author is thought to be enlightened, and because reading it might help one to get enlightened, also.
Zen Masters are by definition greatly enlightened -- if you are not greatly enlightened, you cannot be called a Zen Master. The way that enlightenment comes about is not only a topic of great interest in Ch'an literature, but its guiding question.
Figuring out if a person is a Zen Master or not is strictly a matter of accepting what he says about himself and his own enlightenment story (or not), as well as listening to what other Zen Masters have had to say about it (or not). Of special but not absolute importance is a recognition or certification of one Master's awakening by another Master who has been recognized as awakened by a previous Master, &c.
This is because there are no "objective" standards, no fixed rubric to help one determine who is enlightened and who isn't apart from the verbal record. At some point, one must simply "believe" it to be so, or at least, not disbelieve it.
Since Ch'an Buddhist students were already disposed to believe that "sudden enlightenment" is possible, though rare and hard to attain, it stands to reason that they would want to hear as much as possible about how a person who claims to be enlightened managed or happened to become so.
Reputed Zen Masters were customarily sought out even in remote mountain retreats because it was believed that they might have some special talent, method or tactic for enlightening the seeker in his turn. And, in fact, like the fabled swordsmen and kung fu experts in Chinese wu xia movies, each Ch'an Master was renowned for some special style or trick, such as Lin-Chi's shouts or Yunmen's "one word barriers." Though, instead of being used by the Masters to win duels, these special tactics were used solely to jolt students into sudden awakening.
There would be no other reason to seek out Yunmen except the belief that Yunmen is enlightened, and the related commonplace belief that Yunmen may therefore be able to enlighten the seeker, also. After all, Yunmen himself approached his own Master for help in "clarifying the mind" (attaining wu, satori).
So it is quite natural for any Zen student to ask, like Yunmen, How do I clarify "this"? And it would be unreasonable to ask such a question of anybody but a person whom one believes, or at least does not disbelieve, to be enlightened.