Or he has integrity and is keeping his word that he will not reveal his sources. Assange deals with state secrets. He knew going into this business that there would be times when revealing his source would be beneficial to liberty. And certainly he knew that in some cases revealing a source would result in death of innocent people. But he's stayed true to his word.

By the way, re-watch the interview where Assange talks about Seth Rich's murder. He doesn't come out and say it but it is heavily implied that Seth is the source