When Arjuna was feeling bad to fight against Bheeshma and his Guru Dronacharya, Shri Krishna tries to enlighten Arjuna by preaching Geethopadesha when they are in the middle of the war field. What were other doing-especially Kauravas, because the war was already started during that time?
Because that could be the right time to shoot an arrow on Arjuna. And it was a complete waste of time for others.
And when Shri Krishna shows Vishwaroopam to Arjuna, will others also see?