The Bhagavad Gita is special because it is narrated in a very worldly, very practical setting.
Bhagavad Gita is not guru sitting under a shady banyan tree in the tranquil silence of a holy jungle, sermonising to some pliant, obedient, willing student. Neither is the setting tranquil at all, nor is the sermon academic at all, and least of all is the student pliant and willing. It's a real-life setting in which you do not have the luxury of a formal podium, a formal appointment, a formal code of conduct. It's the war chariot, not the temple. There are armies around, not silent idyllic trees and cute animals, and the one being spoken to is a biased, emotional and unwilling listener, not some keen student.
Life and death depend on this discourse in the literal sense, not just figuratively. Literally, life and death depend on this discourse. Therefore, this discourse has so much potency.
