I've recently been thinking about counterfactuals in this case and am now starting to think that Jock and Jane may in fact be from the U.S. The evidence for them being Quebecois tenuous at best, based on one witness who didn't immediately come forward. Some of the physical evidence like the truck stop matches and Jane's southwestern-looking jewelry seem to suggest that they traveled significantly in the United States.
As for why no one seems to be looking for them, I have a few possible speculative ideas. I have seen cases in the past where postmortem reports ended up being significantly "off" about the victim's age. Initial reports estimated that Jock was 18-22 and Jane was 18-20, but NamUs estimates that they could have been older (Jock's age range is 18-30 and Jane's age range is 18-25). Let's assume for a moment that one or both of those is wrong -- suppose Jock was 31 or 32, and Jane was 26-27 and that they met while traveling and therefore weren't known to each other's families. I could imagine a situation where someone doing a cursory missing person search would gloss over a report of a murdered 18-22 year old couple because it doesn't seem like a fit if they're looking for a single man in his early 30s.
The other idea I have--which is admittedly more far-fetched--is that perhaps their killer was someone they knew and trusted. The lack of defensive wounds seems to suggest that they were killed very quickly and may not have even seen it coming. If the killer knew who they were, it possible that they could have perhaps used their identities for a while or maybe even given false information to their families. As a result, that could skew where investigators or the families looked. (ie, suppose the killer used one of their names or credit cards in another state week later. Investigators would then think "last seen in [state]" rather than South Carolina).
Just my speculative guess. I could be totally off on all of the above, but I have a feeling that if this case is ever solved, there are going to be factual details that will come as a surprise.