Good question! In any question of interpretation, if we don't have any authorial notes, we must reason from two or three points: the evidence in the text, knowledge or attitudes the author could assume his readers would have, and any information we might have about the author's life and attitudes.
Given those general ideas, we have textual reasons for two silences on the part of the damned, and cultural reasons for all of them. We're told that Brutus doesn't speak, and that Judas' head is inside the tormenting devil's mouth; these two would not speak. However, I think the cultural reason trumps all: I think Dante's readers would see these sins as unforgivable, and unjustifiable, and so he could not allow them to speak. These are the blackest sins.