I think it has alot to do with some cars having solid-axles in the rear, and having the pipe run down the side of the car, so it's angled straight out of the back.

Cars with independent rear-ends generally have the exhaust pass near the pumpkin of the rear, so the exhaust has to be angled away from the center of the car... unless you wanted two crazy-ass 90* bends. Which isn't good.

.. but that's just my guess.