Because those were the odds estimated by one of Obama's top national-security advisors -- who nevertheless advocated a drone strike instead of a Special Forces mission.
Nearly everyone in the United States agrees that our government was justified in hunting and killing Osama bin Laden, a confessed and unrepentant mass murderer who escaped justice for years after masterminding the most devastating terrorist attack in American history, boasting about it, and promising more of the same. The operation that killed him is the subject of Mark Bowden's book The Finish, a lengthy magazine article by the same author, and several related interviews. So far, reactions to his account have focused on whether the team of Navy Seals who penetrated Bin Laden's compound were justified in shooting him, or ought to have captured him alive. It's an interesting legal and moral question, but hardly the only one raised by Bowden's reporting.
Consider the moments a bit earlier in the chronology, before the Special Forces team was dispatched. Team Obama had intelligence suggesting that Bin Laden might be the tall man they'd spotted walking around the garden in a walled compound. What were the odds that it was him? The national security team gave answers ranging from a 30 percent chance to a 95 percent chance. Most fell somewhere in the middle. Obama finally directed that they should make their decision as if there was a 50 percent chance that the man inside the compound with two other families and numerous children was Bin Laden. Should they blow the whole thing up? Try to hit the man with a small missile that would kill only him, but might miss its target? Do nothing? Or send a team to gain entry into the compound to see who and what was inside?