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Why do military aircraft have a lower landing gear than civilian? I have in mind the difference between a Lockheed C-130 Hercules and a Boeing or Airbus.
The first possibility that jumps to mind is that one is more comfortable than the other, which is more robust. But would an aircraft designer reduce robustness to provide more comfort?
There is no difference between civil and military aircraft. Not the least because the civil and military transport aircraft are actually the same models. Many military planes have civil variants like Lockheed C-130/L-100, other military planes are derived from civil ones, e.g. C-135 is B707.
The difference depends on where the wings and engines are placed. All the aircraft actually have similar clearance from the ground. Just on some the lowest point is fuselage, on others it's the engines hanging down from the wings and on propeller driven ones it is often the propellers.
Cargo planes (like C-130, C-5, An-124 or An-224) are often built with high wings, so the fuselage is lower and therefore easier to load. This is also concern for regional airliners as they are designed to carry their own stairs. That's why BAe146, ATR-42 or Dash-8 are high-wing and other like Bombardier CRJ series have tail-mounted engines so they can also be lower. On the other hand low or mid-wing designs with wing-mounted engines have better aerodynamic properties, which is why that construction prevails for large airlines.
Easier to Load
As stated earlier, this is your primary reason. You have to be able to load in cargo from the ground. If you want to get troops and vehicles in fast to somewhere with minimum equipment, you want to be able to roll them of the aircraft, and the lower the aircraft, the easier this is done.
Other considerations:
Shorter gear is stronger
The longer you make the gear, the more it will bend. Bad news if you're trying to land on rough ground.
More Cargo space
The shorter you make the gear, the less trouble you have finding space for it inside the aircraft, where you want to maximise the amount of space you have for cargo, especially for vehicles and the like. The anatomy of a cargo aircraft is that they are typically hollow inside, unlike passenger aircraft where they fold up into the center section.
Landing gear is heavy
Landing gear is very heavy, since its got to support the weight of the entire aircraft above. The shorter you make it- the lighter the aircraft gets.
Better use of ground effect
The low gear may bring the aircraft a little bit closer to the ground, maximising ground effect and lowering your landing speed, again very ideal for rough terrain.
Note: These don't only apply to military aircraft, but to a lot of commericial ones as well, since the benefits are the same. Look at the Dash-8, ATR 42/72, BAe 146 along with the rear-engined planes, and they all try to do this for these reasons, although ease of cargo loading might not come first on the list.
For the aircraft you use as examples the aircraft height above the ground (landing gear height) is mostly a function of use-case and clearances.
For the Boeing and Airbus airplanes you mention, the gear need be long to accommodate the engines hanging under the wings as well as the minimum bank angle needed to avoid wing strikes.
On the other hand, the C-130 has the engines mounted up on a high wing and the aircraft needs to handle big slow heavy things like tanks being driven into the cargo. The need of the tanks specifically probably mandates the interior floor be pretty close to the ground, or you might need extremely long ramps to for the tank to ascend (and these would pose their own problems).
C-130 Addendum
Purpose-designed for cargo - rather novel at the time back then, perhaps. Look at WWII films of the C-47 or Berlin Airlift film of C-54's. How the heck do they get anything in there?
Designed around a "standard truck bed height" floor and a "standard railroad box car" dimensions - 10' x 10'.
The short landing gear derives from the basic design goals. Further, the wheels are in tandem - one behind the other vice side by side. And the tires are fat. These aid operating out of unimproved/dirt fields. The leading tire "plows the way" so to speak for the rear tire. Fat tires lowers the "per square foot" weight on the tires.
The longer you make the gear, the more it will bend. Bad news if you're trying to land on rough ground.
Ironically the C-130 landing gear is wimpy when it comes to side loads. It does not have adequate side-bracing. Straight ahead, we can pretty much plop that puppy on the ground relatively aggressively. But, during taxi, stopping while still in a turn is a write-up and requires maintenance inspection prior to flight.
The first possibility that jumps to mind is that one is more comfortable than the other,
The C-130 is designed for utility, by masochists. It is painfully loud, too cold and too hot at the same time, vibrates like the inside of a jack hammer; and if the the prop synchro-phaser ever fails will drive you insane in short order.
The biggest factor in landing gear length is going to be how the engine(s) is/are mounted and ensuring they have clearance. The C-130 and turbo-prop commuter airliners (Embraer and Bombardier) have the engines in line with the bottom of the wing which is attached to the top of the fuselage. Large airliners (Airbus and Boeing) have engines slung well under the wings which attach near the bottom of the fuselage. The turbo props get a lot of clearance from the height of the fuselage while the jets rely on longer gear legs.
