Possibly instructed by Lawyer not to for reasons we don't know.
They have long passed the point where they could make a statement to garner public sympathy. At this point, with all the questions in the media swirling around them, and any attempt to answer would only lead to more questions and scrutiny. The ONLY way the public would ever feel sympathetic to BL's parents is if it somehow turned out he wasn't the killer and they went through all of this for nothing (pretty sure this will never happen). So, with so little chance to save face, the next best thing to do is just shut up.
Better to be thought a guilty party than to open your mouth and remove all all doubt.