Seymour Hersh's sources for My Lai gave their names and went on the record. The soldiers who revealed it went on TV and testified before Congress. I'm pretty sure that he had additional anonymous sources, but to get it published he had to be able to back it up.
Abu Ghraib was uncovered by Amnesty International and the international Red Cross and they had tons of photos.
By the time Hersh first wrote about Abu Ghraib there had already been reporting on abuses by Amnesty International, and several days earlier some of the photos had been shown on 60 Minutes (as he mentions in the piece). The piece cites an Army report, credited to two named generals, and names seven suspects, six of whom were under active criminal prosecution. His report was published (and vetted!) by The New Yorker. The difference between that piece and this one in terms of sourcing and credibility is like night and day.
Nobody much wanted to talk on the record about My Lai or Abu Ghraib when it happened either.