I suspect the answer here is that the format is written as "RAW" to match other common file format names which are acronyms such as JPEG, GIF, MOV, MPEG, etc. Nikon, Canon, Pentax, Hasselblad, Olympus, Sony, and many other manufacturers all write the format in all caps, so at this point it's a de facto standard. There is a distinction to be made between a raw image file in the "RAW" sense, i.e. image data unprocessed by the device, and a raw image file in the sense of "straight from the camera, not altered using image manipulation software", and I think writing the file format as "RAW" rather than "raw" helps to make this clear. When you write it in all caps you're indicating a specific file format, even if that format changes from one device or vendor to another.

Alan Shutko points out in his comment below that older cameras used file systems that only allowed uppercase filenames, like "IMG_0001.RAW". This is an excellent point and seems a likely reason that the format was initially written in all caps.