Digital Cable Question

I wrote in a post last week about breaking my Morrow Digital Cable by accident. Mike Morrow very kindly repaired the cord and mailed it back to me. While I was getting ready to replace the cable into my system, my Wife was looking at it and asked me "why can't you just use a regular interconnect cable for a digital cable? They both have the same connections". I opened my mouth to respond to her, and then realized that I have no idea what differentiates a digital cable from a standard RCA interconnect cable. Can someone please enlighten me?