Airbus A330, which is larger than the 767, could also carry soldiers and cargo seemed to be a selling point. Uh, why would that be a selling point when the USAF already has more C-17 than it wants or needs?
So they only need the one plane to deploy as opposed to a freighter AND a tanker. The sophisticated European Air Forces have been doing this for years as it significantly reduces costs "overall" using the one aircraft as opposed to a quartet.