Everyone loves Cheese Fries, but you can't get them at Burger King or McDonalds.
It sounds like a no-brainer to me. If people love cheese... and they love French Fries, you'd think you'd see Cheese Fries on the menu of just about every restaurant that serves fries. They already have the ingredients, so why not combine them?
Wendy's has a history of offering a variety of toppings for their fries, including not only cheese, but chili and other toppings, but Burger King and McDonalds have never offered anything, which is odd, because it's a simple way for them to sell an upgrade using ingredients they already have on hand.
To me the best Cheese Fries are at Nathan's Famous Hot Dog Restaurants. I rarely order fries from them any other way.