I guess the problem is that it can be difficult to get people to install QR reader on their phones. I doubt that many even know how to operate app stores on their phones. QR works a lot better in a technical context (ie. you target just certain kind of people). It's not that mainstream yet although it's ancient tech. Hardware has fortunately caught up. I bet QR would be more approachable if it would just work out of box. As far as I can see that's the only major problem before it can really break through. If you can tell people just to "point and shoot", it will work. Anything extra and they just might not bother or know how to.