It is said that if you want to know a person's heart, listen to what she or he says when she, he, knows that she, he, is going to die.
Time is short.
There is no time for the extraneous. People talk about what is most on their mind, what is most important to them. During the last supper Jesus had only a few brief and precious hours left with I His disciples. Their journey of three years together was coming to an end, and their small group was about to be rocked with the death of its leader. There certainly were many things that Jesus could have talked about with His disciples.