I am going to explain "Why the knowledge of Gita is confidential" on the basis of another similar shloka given in the 15 th chapter i.e. last Shloka of 15th chapter ,and commentaries written on this shloka by scholars including Adi Shankaracharya.
There are two different point of views (meanings) given , with which we are going to look at your query.
Now let us look at last Shloka of 15th chapter -
इति गुह्यतमं शास्त्रमिदमुक्तं मयाऽनघ।
iti guhya-tamaṁ śāstram
idam uktaṁ mayānagha
kṛta-kṛtyaś ca bhārata
15.20 Thus, this most secret science has been taught by Me, O sinless one; on knowing this, a man becomes wise, and all his duties are
accomplished, O Arjuna.BG 15.20
Point 1- English Translation Of Sri Shankaracharya's Sanskrit Commentary By Swami Gambirananda.
15.20 This guhyatamam, most secret, i.e. most mystical;- what is that?-sastram, scripture-. Although the Gita as a whole is spoken of
as the scripture, still this chapter itself is here referred to as
such, and this for eulogy as is evident from the context. For, not
only has the entire meaning of the scripture Gita been stated here in
brief, but the whole purport of the Vedas also has been comprehended
here. And it has been said, 'He who realizes it is a knower of the
Vedas' (1), 'I alone am the object to be known through all the Vedas'
(15). (Thus, this most secret scripture) BG 15.20
What Adi Shankaracharya mean is-:
Here the word "Secret" does not mean that this knowledge is hidden , since Shree Krishna revealed it ,but secret in the sense that the person (dvija) ,who will know this will become knower of Vedas.
It's said in the commentary that "why the knowledge of this chapter is secret" It's because the entire meaning of Gita and hence the whole purport of Vedas also described here.
A dvija who knows this becames learned in Vedas ,if he is perfectly following his Dharma and living his life according to scriptures.
That's the real meaning of word secret here.
Point -2 :
English Commentary By Swami Sivananda.
The second point is why this knowledge is Ghuya (Secret). It's because it should be to handed down to the person (dvija) who is only eligible to receive it.
By using the word Anagha? also indicates that the Guru who is a knower
of Brahman should instruct the most profound secret (the science of
the Self) only to alified persons who are free from impurities of
the heart or tossing of the mind? who are calm and endowed with the
four means of salvation. The man of impure mind will not be able to
grasp the truth. The sinful man with his perverted intellect will
distort the truth and thus pave the way for the destruction of himself
and his followers.BG 15.20
Conclusion - -So from these two points we can say that this knowledge
is secret and confidential in the sense that It's the purport of the
entire vedas and the person who knows thus become knower of Vedas. And
this secret knowledge only to be revealed to a eligible person , who
is pure from heart ,sinless and perfectly following Dharma.So the
knowledge of Gita is confidential in the senses of these two points.