In the olden days, many photography contest would ask you for your NEGATIVE, for every image you submitted. On the one hand, this was to ensure the authenticity of images. On the other hand it was a stinkin' idea since these negatives would most of the time get lost, so that your prize-winner ended up a hapax legomenon (one of a kind).
In them modern times, you ask for camera files in order to reduce the probability of cheating, using EXIF and other file metadata to authenticate the photo. With a scan or a retouched image, you have none of this, so your image could be "borrowed" from somewhere else. As Nikon does not feel like evaluating negatives/slides anymore, they're only accepting camera files.
That said, EXIF can be fudged by people in the know, just like negatives could be duplicated by people in the know.
But people who are so technically proficient usually don't participate to photography contests.