It is not important where Krishna gave updesh to Arjuna but why Krishna gave updesh to Arjuna.
Lord Krishna gave updesh to Arjuna because he asked for it. All discourses by Lord Krishna were perennial but how many people world over heard it. The numbers can be counted on fingers.
For travelers of spiritual path, seekers the voice of Lord Krishna from within can be heard when we were 100% truthful and succeeded demolishing our ego in totality.
The world 100% means 100%, anything less would not do. For seekers to reach state of absolute purity as a soul atman, our undertaking spiritual journey is necessitated. We had lots of queries within that need answers. Queries life who I am, from where I came, what is the goal of my life, my relationship with God, our relationship with world of souls atmans, meaning of karma, meaning of karma, importance of truthfulness and so on.
To reach state of enlightenment we have to dissolve our queries in totality. For this guidance from Lord Krishna gets necessitated. When we were 100% truthful the voice of Lord Krishna guided us from within. This is what inner journey is all about. Indulging in spirituality when we covered inner journey, we could clearly hear dictates of Lord Krishna coming from within our heart. And the biggest lesson Lord Krishna gives is we are primarily a soul atman, the spirit.
As a spirit we manifest human form to remove dross impurities within. And starts spiritual journey of a seeker like Arjuna! Indulging in spiritual pursuits of life day and night when Arjuna was confronting Kaurava army he all of a sudden reached state of absolute purity. It was then Lord Krishna outpoured sermon of Bhagavad Gita. After gaining jnana wisdom from Lord Krishna, Arjuna finally prepared himself to fight the enemy Kauravas.
Earlier being a spiritual person Arjuna was feigning cowardice and did not want to fight the enemy Kauravas consisting of his close cousins, relatives and friends even maternal grandfather Bhishma Pitamah and guru Dronacharya. It was Lord Krishna who made Arjuna realize the shallowness of earthly life. The actual master and controller of human life was spirit, soul atman within and as souls atmans were indestructible, in killing Kauravas he would not be killing anybody or committing a sin. He would only be doing his duty towards his countrymen and soldiers.
Bhagavad Gita allegorically is a treatise that has to be indulged by spiritual travelers desirous of reaching state of enlightenment followed by moksha salvation.
Every single seeker, traveler of spiritual path can invoke messages from Lord Krishna by becoming 100% truthful and demolishing ego in totality.