As stated in Bhagavad Gita Lord Krishna says spiritual teachings must be disseminated only to a rightful candidate, never one who was disinterested. In present times the whole world appears spiritually externally, outwardly but on the inside none wants to indulge in spirituality. All followed religious practices (rituals) that were totally meaningless on path of pure spirituality.
Indulging in religion (rituals) one never reaches God.
Both Krishna and Arjuna came in Dwapar Yuga, in times of great strife. The society had seen its lowest ebb. The only solution was a gigantic battle to cleanse the society of its ills. But for this a seeker, traveler of spiritual path with competence of Swami Vivekananda was necessitated. Unless one was on the verge of gaining enlightenment, teachings of Bhagavad Gita could not be imparted to one. That was how God Almighty ordained spiritual journey to be.
Amongst the fight Pandavas and also the entire community Arjuna was the only right candidate who could have assimilated teachings of Bhagavad Gita by Lord Krishna as he was already indulging in spirituality to reach God in his lifetime.
Imparting spiritual wisdom to inept in contemplation (chintan) would never work out. Only one who had mastered delicate art of contemplation (chintan) was the right candidate for assimilating wisdom contained in Bhagavad Gita.
Even during times of Mahavira (24th Tirthankara of Jainism) when after reaching state of enlightenment, he decided to retain his body to give back to society what he learnt on spiritual path, there was none in the world to assimilate his teachings. After a time gap of about 1 1/2 year a seeker Gautam gani finally approached him for some clarifications. It was then the wisdom of Mahavira spread to masses through Gautam gani.
If Swami Vivekananda was not there, who would have spread teachings of Sri Ramakrishna Paramhansa to entire world community!
During times of Krishna it was only Arjuna competent to do so. The moment Arjuna realized he was primarily a spiritual traveler, a spirit manifesting human form he decided to fight his enemy head-on. Before that he was feigning cowardice not wanting to fight a war with his cousins, close relatives and friends including maternal grandfather Bhishma Pitamah and Guru Dronacharya.
When it dawned on Arjuna that the human form was only a means to an end, and our soul atman within was the absolute master and controller of life on mother earth, he changed his perception. This happened because he scaled higher levels of consciousness indulging in spirituality. At that level he realized he was only a pawn in hands of God Almighty, a caretaker… trustee doing bidding of God Almighty all the time.
Indulging in nishkama karma yoga Arjuna finally indulged himself in the battle of Mahabharata, a rightful battle to re-establish Dharma (righteousness) against evil forces of adharma (lawlessness).
Allegorically, Lord Krishna is soul atman existing within human body. Contemplating seriously Arjuna finally realized that feigning cowardice when facing enemy was not the solution. As Arjuna represented his countrymen and soldiers, it was his Dharma, duty to fight the enemy head-on.
In the battle of Mahabharata Arjuna realized self, realized god and in that very situation not attaching self to fruits of karma performed he indulged in the battle of Mahabharata.