My opinion only:
I believe the C5 was designed with a high wing to allow lower fuselage sill height for cargo loading. The limiting factor for obtaining low loading height on a low wing aircraft is the engine position so by utilising a high wing configuration the designers ensure the engines have sufficient ground clearance while still achieving the lower sill heights for cargo loading. The lower heights allow vehicular loading with the integeral ramps rather than relying on additional equipment.
I hope this helps!
TH