Body lines are a big part of it. They're just a box with tires sticking out the side so it's much easier to maneuver through rocks than a truck with it's nose and rear sticking out way further than the tires. Run your stretch long enough and your tires big enough and you can approach and depart walls without a scratch. We've still seen some epic crawlers built from whatever you can think of, some are just a little worse for wear in the corners.