Depends on how thin the pizza is. Here in California, we have thick pizzas. Folding them in half doesn't work all that well because the crust is too bulky and stiff. The pizzas already stay mostly flat, so the toppings aren't going anywhere. Over on the East Coast, pizzas are much thinner. Folding a pizza is the best way to keep it from flopping around like a fish out of water. A floppy pizza has a hard time retaining its toppings, so folding it in half makes perfect sense.