I can think of several reasons. They may be very private people and uncomfortable with answering dumb questions from reporters. And the info that the public needs to know, her description and the clothes she was wearing has already been put out by LE, no need to repeat it from the parents. They could go on camera every day and plead for her safe return, but I can't think of one abductor who ever released a child or brought them home because of a parent's tearful plea in the media, can you? And sadly, statistics say that by the time the parents speak out, it's too late, anyway, the child is already dead within a few hours after they were kidnapped.
Respectfully, it does not have to mean they're hiding something... although they could be... but there is no rule that says they have to speak out. And in following so many of these cases, I have come to believe that the less they say in public the better. Look how much criticism the parents have got so far just from that one appearance. Anything they say to the media is going to be sliced and diced to kingdom come, but it won't help find this little girl.