The larger 777 can lift much more cargo and carry more passengers than the 767 and fly them to a farther destination. Much better for deployments when it counts. The avionics are close enough not to be a big factor.
Evidently, that was not what the Air Force desired.
The entire passenger area of the 767 is available for cargo/passengers. All of the fuel in the tanker is stored in the area formerly used for baggage/cargo.