The only reason that I can see is that its short term thinking, they can't afford another team under the current TV deal so expansion wont happen until there is a new one. This is an opportunity to move a team from a small market into a big one and save a bit of money while they are at it. It's just the nix bad luck that all the other teams have 20 year contracts.
There is no doubt that they will have to look at trying another NZ team in the future at some point, unless they expand into Asia which has been talked about before.
I hope its a bitter failure and the fudgeing league has to fold because of it, then the top Australian teams decide to break away and Join the newly profesional ASB Premiership.