I don't know.
The White House says that President Obama concluded that the photographs' release could bring harm to United States troops. His spokesman, Robert Gibbs, very carefully and slowly offered a further justification: that those who take photographs of abused detainees in the future might be harmed by the release of the photographs because a precedent would be set. Also, the release of the photographs will not enhance anyone's understanding of the specific cases. I'm not sure I understand what Gibbs means, or why these arguments suddenly occurred to Obama.
ABC's Jake Tapper has good insight into the decision-making process.
Here's what I told CBS News radio:
I think it shows how the presidency can change a person's mind about the tradeoffs between transparency and what's best for the country. Obama came into office promising to be more transparent than any president before him - and this was a big campaign issue - but he has slowly come to realize that transparency without context can be costly. That's not an excuse for what he did, but it explains why he is open to changing his mind in these circumstances.
I think there's a legal issue here too: Obama is defending a principle that allows the government to decide what information is harmful to national security interests... and not a court or congress. That's an executive prerogative he wants to uphold.
That said, it's hard to square his decision here with his decision to release those DoJ torture memos, which were very inflammatory. The same arguments can apply. I think the White House would put on a neuroscientist's hat and argue that visual depictions of torture are potentially more harmful than banal legal language describing the depicted practices.