Somebody once asked why fast food places don't normally offer pieces of fruit as part of their menus. There are some, mind you; for example, Subway sells bags of apple wedges. By and large though, they don't.
I think there are several reasons for this. One is that fruit doesn't tend to keep very well. Apples and pears tend to get brown as they age, for example. Fruit also tends to be expensive in comparison to, say, a bag of potato chips.
Ultimately though, the main reason is demand. It's why KFC stopped selling rotisserie chicken back in 1992. It's why the McLean didn't take off, and why I sometimes find it hard to get a grilled chicken sandwich at McDonald's. Your typical fast food customer doesn't care much about nutrition, and so he'd probably prefer a cookie over a bag of fruit.
I'm not just speculating, mind you. I say this based on my observations of fast food customers. Also, the manager of a fast food restaurant once told me that they sell close to half a ton of french fries in a week, but only about ten cups of cut fruit. That's America for you.