Subway is the cheapest of the franchises in terms of buy-in and they offer no territorial protection for the franchisee, so if another mope wants to open one across the street, they're okay with that. This is why you can stand on nearly any corner in the Loop and be a few hundred feet from a Subway in nearly any direction. Profits per location end up being low relative to other franchises (you can make over $1million from a good McDonalds location, maybe a tenth of that at a great Subway).
And there's nothing sadder than walking down a medieval lane in Europe and happening upon a Subway. I'd gladly take a European McDonalds, at least they try to build a presentable location.