I think nobody will disagree that Belfour and Roy were the 2 best canadians goalies in the 90's, Brodeur was great but not yet in his prime. With that in mind it is crazy to think that in the big international competitions only one time one of the two was a starter and that was Roy in the 98 Olympics. Why Belfour or Roy weren't the starters in the 1996 World Cup, why Roy was not even selected to the 1991 Canada Cup, and why Belfour was not a backup in the 1998 Olympics?
First of all, if we're talking about "big international competitions" and the 90's, there's really only three tournaments to talk about: 1991, 1996, and 1998; so your entire premise isn't
that astonishing.
To answer your questions though, I'd say a big part of it goes back to Hockey Canada and the guys in charge; Aside from a few exceptions, Hockey Canada tends to select players who have represented Canada before (specifically World Championships) and who they're familiar with. That's probably why we see both Ranford and Joseph in 1996 for example (familiarity through Glen Sather/Oilers, both played in previous WC's, Ranford in the last Canada Cup).
As for Belfour and Roy specifically, I believe Roy was recovering from surgery in 1991 and Belfour was still a rookie, so it's understandable that they went with a more proven tandem. Belfour also suffered from having a "choker" label for the first half of his career which maybe explains why he was passed over for the best-on-best tournaments.