Not camera manufacturers, but photographers who don't want the large file sizes of 16bpc TIFF files.
Cameras will not add support before photoshop does. If CS6 does not support JPEG XR, we can forget about the format for another 2 years.
The web has more fundamental problems than image compression. Many browsers (including IE9, chrome) do not have working color management. I.e. these browsers cannot even display JPEG (or any image or colored object) yet.