Having done YVR-LHR-YVR's in the past month, I can say that at least half of the business class pax were one off's (using my best prejudicial skills: fumble factor, no idea how anything works or where it is, excitement). The SS lounge gives AC a distinct value proposition advantage over competitors and rewards the pax that pay the most for that particular flight.
No guests allowed unless in paid business class hence earning their own seat at the table.

If NZ were to have such a lounge and open it up to E's on 086 metal but any class, it would require a much larger lounge and reward bottom feeding spending behaviour. The SS lounge is to reward people paying the most per mile.
I suspect NZ has many E's who pay for business class flights, such a lounge is a way to reward and thank those people and to encourage them to buy NZ business class fares as opposed to UA/AA/Delta/AC/EK/etc.

For YVR-AKL-YVR, one would need nested returns to have AC's SS going down and NZ's coming back to YVR!