Mostly, the high-wing and landing gear configuration on many cargo aircraft designs is intended to facilitate a low loading floor and the ability to load and unload the aircraft on rough fields without special ground equipment (e.g. by means of a ramp integrated with the the rear doors of the aircraft like a C-130 or with a deployable forward ramp and kneeling front gear like the An-124.) Another benefit is providing extra ground clearance for the engines to reduce foreign object ingestion.
Everything else flows from this. The wing spar and structural supports needed in a cantilever monoplane (i.e. not a biplane with a truss structure) take up a fair amount of room. By moving these structural elements to the top of the aircraft, it's possible to get a low floor and still hang engines from pylons beneath the wings. The wings themselves will be angled downward from root to tip (known as anhedral) to partly counteract the pendulum stability caused by having a center of gravity so far below the center of pressure.
Passenger aircraft do not usually use this configuration because it is heavier - the sides of the fuselage must be built stronger to support the load, compared to a low-wing configuration. Weight costs performance costs fuel costs money, so you don't build a large aircraft like this unless it has to operate at fields without ground equipment.
Everything else flows from this. The wing spar and structural supports needed in a cantilever monoplane (i.e. not a biplane with a truss structure) take up a fair amount of room. By moving these structural elements to the top of the aircraft, it's possible to get a low floor and still hang engines from pylons beneath the wings. The wings themselves will be angled downward from root to tip (known as anhedral) to partly counteract the pendulum stability caused by having a center of gravity so far below the center of pressure.
Passenger aircraft do not usually use this configuration because it is heavier - the sides of the fuselage must be built stronger to support the load, compared to a low-wing configuration. Weight costs performance costs fuel costs money, so you don't build a large aircraft like this unless it has to operate at fields without ground equipment.