For the obvious reasons of character we would say that this is why. Duryodhana was full of ego while Arjuna was not since he surrendered himself to the Lord. Arjuna had the mind and heart of someone that was worthy of Shree Krishna’s revelation. What kind of mind surrenders?

When he was under training to shoot the arrow and his Guru said to shoot the bird only he among the warriors was so focused that he said he only saw the eye of the bird, whereas the others saw the branches and other things. His mind was not wavering, it was focused. He was steady. A meditative mind is steady and focused.
How many of us can see the divine in others? Only he had the ability to see Lord Krishna as a friend and as the divine. He knew Shree Krishna was divine and got the whole truth when Shree Krishna revealed his other forms. Most people only know the roles that they are in. We have a different relationships with each person: mother, daughter, aunt, grandmother, friend, in-laws, co-worker, Facebook friend, priest or pundit etc. For each relationship we begin to play a role of what we talk about and the extent of closeness we have with that person. So we have different levels of love and respect according to the acquaintances. So if God decided to manifest in a human form in front of you would you be able to recognize him? If he showed up us someone you know, you will only know him as that acquaintance.
So how would you know? Will you rely on someone else to make the observation and then you concur or would you know yourself? You might be spiritually aware from experiencing the spirit within yourself to recognize it. If you have an open mind and an open heart and have a true connection to God then you will feel it so you might be spiritually aware enough.

An open mind and a open heart is necessary and the connection between the brain and the heart in the human body is very important. Meditation on any form of God with love will begin your journey to know spirit for this is the way to expand your heart. Sitting down and observing your thoughts leave your system will cause a more open mind as it becomes fresh with awareness instead of a loaded database running SQL queries.
Then you can become like Arjuna. Arjuna was able to see his Lord Krishna in more than one role and have such a relationship. He was able to surrender. There are only few people capable of doing this in the world. I only know of one so far.

The Bhagavad Gita is always valid in every age since the war within human beings is always going on and expels into relationships, society and countries. Some of the esoteric meanings of the Bhagavad Gita are explained by Yogannanda.

Our ways of acknowledging God might be different but it’s nice to share the planet with you.