A state-of-the-art aircraft like the 787 comes at a premium.  The Airforce prefers to save it's money for combat aircraft.  For support aircraft, they prefer to use older, cheaper designs.

Plus the fact that support aircraft really don't need to be state of the art, since they tend to operate in relatively safe areas.  Why spend an extra $100 million to make a cargo aircraft stealthy, if you never send them near an enemy air defense radar?