Structurally, it's easier for something to hang (well in places that have gravity as a constant) than it is to prop up. You could mount something above the wing

Structurally, it's easier for something to hang (well in places that have gravity as a constant) than it is to prop up. You could mount something above the wings but than you would need additional hardpoints to keep it from falling over and smashing into the wing. If it's hanging you only need one hardpoint.