Well, a straight 6 has an enormous crankshaft (6 cylinders long while a v6 has a short crankshaft at 3 cylinders long) which works as an enormous flywheel. That enormous flywheel can store enormous amounts of energy. That enormous amount of energy means an enormous amount of torque. Most I6's have longer strokes than v6's and longer rod-stroke ratios. These also contribute to higher torque numbers.