Only because the Navy owns the Marine Corps. All F-35s for the Navy are F-35C and all F-35s for the Marine Corps are F-35B, and all F-35s for the Air Force are F-35A.
The British Royal Navy are purchasing F-35Bs for both their Air Force and Navy.
The Marines are getting F-35Cs too, to fulfill their commitment to having some squadrons integrated into carrier airwings.