Why did Airbus not offer a cargo door in the USAF competition?
They did.
KC-45 Aerial Refuelling Tanker Aircraft - Airforce Technology
Cargo
The main deck can carry 280 passengers or 26 463l pallets. Loading and unloading is through a 141in x 100in cargo door. The lower deck can carry an additional six pallets. The KC-45 cargo loading system is supplied by AAR Cargo Systems which is based in Livonia, Michigan.