Answer:
Since the standard square black and white QR code is used by most businesses when they include them in ads or on products, many people consider it ugly or unattractive. They are similar to the standard bar code on product or food boxes, the kinds that get scanned at the check-out counter. Originally created for factory use in Japan, they all look alike except to a computerized "reader" program.
Therefore, more businesses or anyone using QR codes will opt to have a unique look to their code if possible - something that will stand out on the printed page. This might include the use of colors or illustrations as part of the code. Almost anything besides the black and white box is an improvement.
The end result is that a reader who sees a typical ad page will notice that QR code more easily as it won't get buried or hidden inside all the other material on the page. If that QR code has a specially unique and customized look, a user is more likely to scan it with their smart phone assuming that the message will be more professional or valuable.
This assumption is usually correct for the same reason that an attractive website will hold readers' attention longer. Any business that has invested in creating a professional website would naturally opt to have an equally attractive QR code if possible. However, because QR codes in general are still fairly new in the U.S. compared to Asia, for example, customized and colorful QR codes are currently seldom seen.