The main reason is that the design concept is different, the straight pipe exhausts smoothly, and the curved one blows the ground dust directly, which can cause unnecessary blurred vision to the rear car.
Some option packages include visible "chrome" tailpipes, so they are usually straight. If an option package doesn't include a chrome tip, the somewhat rusty-looking pipe sticking out of the back makes the tip curve downward looking a bit ugly. Admittedly, sometimes they bend the tip to keep the exhaust from fouling the bumper, but none of this affects performance!
There have been reports of CO intrusion, in many cases attributed to retrofitting of police equipment, different exhaust tip configurations may be the result of trying to reduce this problem.