Why? Because while gory photographs would have inflamed some jihadists and wannabes, I believe they would have disillusioned and deflated others. A heroic myth of invulnerability had been built around bin Laden. He was supposed to have cheated death while fighting the Russians in Afghanistan, walking tall through fields of fire as the bullets somehow missed. He escaped the Americans who cornered him at Tora Bora. He evaded capture for a decade, despite the best efforts of the West’s spies and soldiers.
Showing him in death would definitively refute any notion that bin Laden enjoyed some kind of divine protection. The myth would die with the man.
It’s also true that photographic evidence would silence most, but not all, of the conspiracy theorists (who are surely putting on their tinfoil hats as we speak).