It was based on "more".
I do not really understand how the whole process was run. What I heard on the broadcast announcement here was that it was because it was a bigger and newer airplane.
"The KC-45A is the tanker of the future," said Gen. Arthur J. Lichte, AMC commander. "It will enable us to carry more fuel and cargo, and allow us the flexibility to refuel any type of receiver on every mission."
If bigger/newer was the major cirterion, why didn't Boeing offer the 777? If bigger/newer was not in the selction criteria, Boeing should get a do-over.