Basically folks in the US are not adopting them as quickly industry would like. Most college aged smart phone users do not use them and do not plan on using them. Why? They don't know what is expected of them and most of the sites they [the QR apps] point to are not optimized for mobile devices. Lastly, people need to be lured into downloading a QR reader app, as most smartphone users think the camera should pick up a QR code and run with it.
Please check out the link --it is a very good article!