Lawyers know that having the defendant testify is risky. They might just say something incriminating. I don't know if Trump just doesn't want to be held accountable (the unconstitutional argument seems to me to be a crock). But if I was a lawyer representing him, I wouldn't want him to show up. If he loses his temper at Republicans who voted to confirm the Electoral College results, he could just lose enough of their votes for a conviction.
