I'm sure the fact that McDonalds is EVERYWHERE is a huge reason for it. A massive install base, cheap food, kids' playground., now you got the awful coffee. I think it's more like an anti-restaurant, you go there when you don't really want to eat but have to eat something, or if you don't have time to eat anything else, or just want to eat without thinking about it. They're also in a lot of airports as well, I've eaten that rather than airline food (though there are some good burger places in some restaurants... there was a GREAT little homemade place in the Bob Hope Airport). It's safe, familiar, ubiquitous, and it's the first thing you think of when you think fast food hamburger. They've got the mental market cornered.
I'm willing to bet that the higher up that chart goes, the more localized the restaurants are.