Machinbird, here is the deal. Most jet aircraft simply have a high hydraulic demand to retract a landing gear, it is a demanding task. Extension is simple, open the door, release and secure. Simple engineering allows for gravity, often springs and a small hydraulic demand from another actuator to assist the gear into extended and locked position. Once this is done simple mechanics of an over center link generally are sufficient to hold it in it's down position yet are generally reinforced by a small hydraulic actuator. The retract actuator has one goal and requires a high demand of force.