At the start of chapter 4 Kṛṣṇa reveals the age of what he has been teaching. I gave this yoga to Vivasvān, the sun, he says. Vivasvān gave it to Manu. Manu gave it to Ikṣvāku. Royal sages passed it hand to hand.
Then, over a long time, it was lost.
Why bring it back now, and why to this man? Kṛṣṇa answers at 4.3. The same ancient teaching I give to you today, because you are my devotee and my friend: bhakto ’si me sakhā ceti.
Not a sage on a mountain. Not a priest. A cousin in a chariot, frightened and confused, who happens to be his friend.
The Gītā wasn’t handed to the wisest man alive. It was handed to a friend, and the friendship is the stated reason.
The verses
BHAGAVAD GĪTĀ · 4.2एवं परम्पराप्राप्तमिमं राजर्षयो विदुः।
स कालेनेह महता योगो नष्टः परन्तप।।
evaṁ paramparā-prāptam imaṁ rājarṣayo viduḥ
sa kāleneha mahatā yogo naṣṭaḥ parantapa
Thus handed down in succession, the royal sages knew this (Karma Yoga). But with a long lapse of time, O Arjuna, that Yoga was lost to the world.
BHAGAVAD GĪTĀ · 4.3स एवायं मया तेऽद्य योगः प्रोक्तः पुरातनः।
भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम्।।
sa evāyaṁ mayā te ’dya yogaḥ proktaḥ purātanaḥ
bhakto ’si me sakhā ceti rahasyaṁ hyetad uttamam
It is the same ancient Yoga that I am now teaching you, for you are My devotee and My friend. For, this is a supreme mystery.
Going deeper
Chapter 4 begins with a claim about age. In 4.1 Kṛṣṇa says he taught this imperishable yoga to Vivasvān, Vivasvān taught it to Manu, and Manu declared it to Ikṣvāku. 4.2 says it came down in succession and the royal sages knew it, but that over a long time it was lost to the world. 4.3 answers the obvious question of why it is given now, and to Arjuna: it is the same ancient yoga, taught to you today because you are my devotee and my friend, and this is a supreme mystery. 4.4 is Arjuna's puzzled reply, about how Kṛṣṇa could have taught someone born earlier. The reason the text gives is not learning or rank. It is the relationship, stated in a single phrase. The passage is in chapter 4, and what the Gītā says about devotion collects related verses.