Pdub,
My guess is rather the advantage of having a spacious single cargo hold, whereas with a low-wing aircraft a comparatively high cargo hold floor is required to have enough space for the main spar. The low cargo floor is also advantageous for the operation of aircraft like Antonov 124 or 225 where no external ground equipment is required to load/unload the aircraft. The aircaft is able to retract the landing gear up to a point where the belly almost touches the ground, so there is only a small height difference between the apron and the cargo hold floor which can easily be handled by the built-in ramp and the two cranes running in rails on the cargo holds roof.