Because you have to listen to testimoney that could be very upsetting and look at pictures that could be very disturbing. After all they are human beings having to "judge" the actions of other human beings who caused harm to someone or something. Things that stay with you decades later.
Has that ever happened to you too? Any cases that haunt you?
I don't know if I could do it. I don't know if I have the "right stuff" to take it, analyze it, think about it and conclude anything.