Because, my friend, waterboarding is torture, and a war crime, and it reduced the moral authority of the United States to slightly above that of the miscreants who engage in public beheadings. Allowing it caused hundreds of actual deaths, extended the war, made the actual process of keeping the peace significantly harder, and set a poor moral standard for the rest of the world. I know, minor considerations.
Hmm... interesting. Drone strikes that routinely kill innocent people throughout the region, all done with no risk to those killing remotely has somehow not reduced US "moral authority" or caused "hundreds of actual deaths"? "Shock and Awe" macho posturing, bombing, and killing did not do so as well? Atrocities committed by soldiers, pilots, and other US personnel did not reduce US "moral authority"? Abu Ghraib's abuses somehow failed to reduce US "moral authority" or cause "hundreds of actual deaths"? But, instead, you single out the waterboarding torture of a few probable bad guys from a region that is noted for torture and brutality and claim it did
reduce "US moral authority" and did cause "hundreds of actual deaths"? I am doubtful that it had such an impact.
By the way, referring to maintaining a "peace" that has been elusive at best, and suggesting that the "war" has ended is, in my opinion, seriously wishful thinking.