Why is it 说明 and not 告诉 within 而自知之明之所以“贵”，则[blank]人是多么不容易自知?

Question

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:

说明、叙述、告诉、抱怨

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 (shuō​míng​) to explain / to illustrate / to indicate / to show / to prove / explanation / directions / caption / CL: 個｜个

CC-CEDICT: 告诉 (gào​su​) 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?

Answer 1

说明 isn't followed by an object.

For example, 说明人是多么不容易自知 = it explains how people rarely know themselves (you don't need to state whom you are explaining to)

告诉 needs to be followed by an object.

For example, 告诉人們， 人是多么不容易自知 = it tells people that people rarely know themselves (you need to state whom you are telling to)

Since there's no object in the question, 说明人是多么不容易自知 is the correct answer

告诉(object missing)人是多么不容易自知 is ungrammatical

If the original question wrote: "则 [blank]人們 人是多么不容易自知。" Then "则 [告诉]人們 人是多么不容易自知。" would be the correct answer

Answer 2

而自知之明之所以“贵”，则 [说明] 人是多么不容易自知。

Have a clear understanding of self is deemed a precious personal trait, the sentence "explains" that it is not easy for people to know themself.

In here, 贵 meant 珍贵, which in turn means rare or scarce. The first phrase in the complete sentence was an ancient teaching written in the old language (古文), and the second phrase "explains" the meaning of that teaching using the modern language (白話文).