The KC-X is for aerial refuelling. It is not designed for transport
It also doesn't have the same sorts of take-off and landing requirements as the C-17, which is designed to operate into and out of short (3000 ft), narrow (90 ft) and rough runways.
Are you in common with USAF aircraft designation system? For what do you think the “C” within “KC-X” stands for?
The KC-X is designed for airlift.
System Requirements Document (SRD) for the KC-X
https://www.fbo.gov/utils/view?id=2538827bd21adb15bed0d01bab0f2dde
Within the SRD for KC-X you'll find section 3.2 “Airlift”.
3.2.1 Seamless and Efficient Cargo Operation Within Defense Transportation System
3.2.1.1 The KC-X shall transport cargo and personnel by using only material handling equipment and transportation support processes and procedures employed by other Air Mobility Command (AMC) assets.(MANDATROY)
3.2.1.2 Cargo Handling
3.2.1.2.1 The entire KC-X main cargo compartment shall accommodate an all cargo configuration using 463L pallets. (MANDATORY).
[...]
3.2.3 Payload Combination and Reconfiguration
[...]
3.2.3.1.1 The KC-X shall have a maximum en-route turn time of two (2) hours, 45 minutes for all cargo missions.[...]
A C-17 can carry about 18 463L pallets;
A KC-767NG about 19 and a KC-45 about 32.
For moving troops both KC-X are far better.
“It also doesn't have the same sorts of take-off and landing requirements as the C-17, which is designed to operate into and out of short (3000 ft), narrow (90 ft) and rough runways. “
For most airlift missions these restrictions don't exist. USAF avoids to use C-17 on rough runways. Also C-17 guzzles twice as much fuel as any KC-X.
KC-X can relief C-17 on many missions.