there's a lot of theories, and I shouldn't have stated that as a factual, set-in-stone reason, but since most of those customers have moved largely to chains like Panera and Chipotle, whose whole business model is based on presenting food as healthier, fresher, more "real", and their restaurants as more ethical, more involved in the community and more invested in their employees, there's most likely a big correlation...
also the fact that I regularly hear the typical soccer mom type talk about how they don't go to McDonald's/Burger King (anymore, if they ever did) because of the health (mostly health and sometimes the factory farm type reasons) and anecdotally know so many people who have changed their eating habits and have become much better educated about fast food, a lot of which I think can be attributed to a combination of documentaries/books/exposes about the industry as well as the scientific data/ nutrition studies which have been more in focus since the "obesity crisis" began being spoken of. I think all these factors are undeniable.
And then of course there's what you mention, that people have all become "foodies" and fast food has really grown up, but part of that is the push to eat healthier foods and to not just eat mindlessly. Not that it's doing much good, but still...
eta: most of all, you can just feel it. there's a huge amount of shame in eating fast food (well, McD's/Burger King/Taco Bell/KFC) now that wasn't there 10 or 15 years ago.
hope this signifies the beginning of the end of an era. Not that it will make much difference but it does go to show how population being educated about the practices and quality (lack thereof) of a particular company and their products can make big changes.
