I would like our colleagues from Health Canada to comment, but my understanding of why we tend to use “advance requests” relates to the fact that, should it one day be something that Parliament decides to do, it might be outside of the advance directive regimes that the provinces have set up for other purposes, meaning that it might be a kind of stand-alone regime.
Also, I don't think “advance directives” is the common term used in all jurisdictions across the country. I think there was some desire to use slightly more general language, so that we weren't giving the sense that people were being boxed in to the legislative advance directive regimes under provincial governments.