It makes sense considering his personality: https://www.youtube.com/watch?v=5CPs7f7kYHA. Observe his body language in the first few minutes--the way he sits, where he puts his hands. He's a classic bookish introvert. He opens up as the interview continues, feeding off the interviewer who is a former law clerk and personal friend. But you can see why he might be reluctant to jump into the fray of oral arguments, where people are rushing to get a word in edgewise. (As an aside, I recommend anyone to watch the entire video. It gives a great deal of insight into his personality and thought process.)