Why? Because it is a symbol of masculinity in the South and many men would never ever drive a car. Their grandfather had a truck, father had a truck, and they were given a truck as their first vehicle. It would have made sense for the grandparent on the farm, but if you sit in front of a computer 8 hours a day and live in a suburban neighborhood...you probably don't need a truck.
With that being said, there are folks down here that still need trucks for work and recreational activities like hunting and towing a boat.