SJD-Canadian cities in UA business class - why Z class not available on all routes?

In Feb, we will be flying from YOW to SDJ, and back. We want to come back via YVR, and stopover there.

We wish to fly in business class.

However, "Z" class fares are not available beween SJD and YVR, but they are available between SDJ and YOW. C, D, and J class fares are available on both routes. In may be worth my while to go to YOW first, and then worry about going to YVR from YOW on AC.

The question is why "Z" class fares are not available on the SJD-YVR route, but they are available on SJD-YOW (or SJD-YYZ) route?