Not sure why. If a car had obviously been left there for many days (for example buried under several snow falls) that might have aroused some suspicion. It sounds like they mostly just checked to see if any hotels had missing guests, and if so were those guests cars were left in the lot.

As hikermor said, why didn't the first people who saw the SOS just stop to check it out? Or the resort just have someone go to the location to check? That would be the obvious thing to do. To me that is the real question. I think Kicking Horse has a number of off area guiding operations (guided tours, heli skiing and snow cat skiing). Generally those outfits work closely with the resort. They might have just asked one to stop by the location where the SOS was reported.

As hikermor also said, there is plenty of blame to go around. Not the least of which goes to the couple themselves. Going out on a back country ski tour, without telling anyone where you a headed or when you expect to be back, is not smart. Doing that in an area which you are not familiar with is even less smart.