I read one article that was interesting. It said that the black boxes only have so much recording time. And if the plane was a "zombie" plane as some speculate (due to accident, the pilot making it so, or hijacking) then the black box data would all have been over-written from the "event" and as the plane flew in whatever condition it was in for several hours until it ran out of fuel, that's all the info they would have and nothing on the process that led up to that.
Hard to confirm this though, it was a Yahoo article!
