Now that places like Subway or other gourmet sandwich shops are also fast food, there's no reason for anyone to still be going to McDonald's. If you can get a reasonably healthy meal that also tastes better, why waste time eating grease at the Golden Arches?
The fact that there are 13,000 of them is a bit disconcerting. I guess I never stopped to realize how many of them there are just around where I live. There are at least 7 or 8 all within a ten mile radius of my house. Granted, a couple of those are on a college campus, but that's still way too many.