Two reasons: First, there is a fear that digital images can be editted to make the knife look better than it is. This would be fraud and would discredit the book and its author and publisher.
Second: My own film scanner (not even a professional one) converts 35mm slides into 32MPixel files with twelve-bit-per-channel color depth. On my 2.5GHz PC with four GBytes of RAM, these files bring Photoshop to a crawl. The best common digital cameras do 16MPixels with 10 bit depth and most are not that good. A pro film scanner will be much better than my consumer-grade one. Digital photography has come a long way, but film is still the best.
Keep in mind that what she intends to produce is not a 2-inch picture in a magazine printed on crapy paper with cheap ink (don't get me started on the production quality of Blade, KI, TK, etc.), but a beautiful coffee-table type book with big pictures and quality printing.