A couple of ideas come to mind:
1) Pizza Hut, like all of the other cheap food places, are feeling the "pinch" of their constant promotions. (e.g. - $6.99 specials, etc.) Clearly, they've realized that they can't make any money with these promotions - and by charging, $1.00 extra per order, they can keep the $6.99 promotions going and not have to change the advertising to $7.99.
2) Maybe, they are just charging $1.00 extra in "higher cost" areas where labor is more expensive, or the building rent is higher.
Either way, I haven't ordered delivery pizza from Pizza Hut in over two years. It's great having an "authentic" NY style pizza place within a mile of my house that makes excellent pizza in a brick-lined oven. And for the best part, their cheese and toppings don't have extra sodium like the pizzas from Pizza Hut, Dominos and Papa Johns usually contain.