A HSK5 textbook exercise talks about the saying 人贵有自知之明 (which means something like "self-knowledge is precious"). It writes:
……。把自知称之为“明”，可见自知是一个人智慧的体现。而自知之明之所以“贵”，则 [blank] 人是多么不容易自知。
HSK5上 workbook, p.18 (image)
So the 明 reflects ones intelligence, and 贵 indicates how 自知 difficult to obtain.
In this exercise there are four choices for the blank:
The given answer is 说明, but I'm not sure why. The answer is clearly not 叙述 ("narrate") or 抱怨 ("complain") which are not relevant. But I can't figure out why 告诉 is not correct here.
CC-CEDICT: 说明 (shuōmíng) to explain / to illustrate / to indicate / to show / to prove / explanation / directions / caption / CL: 個｜个
CC-CEDICT: 告诉 (gàosu) to tell / to inform / to let know
I often encounter 这个故事告诉人们 at the end of anecdotes, which is why I chose 告诉 above.
Question: Why is it 说明 and not 告诉 within the above?