Who says that? He hasn't hit arbitration yet, and even if he doesn't keep this up, he's still a good backup.
The question you should be asking is why Tampa Bay decided to get rid of him for Josh Lueke.
Well, at the moment, I think Montero and Jaso are going to be the catchers until Zunino is ready, which may not be for two years. In the meantime, they might use someone like Moore (assuming he stays) or another veteran like Olivo to help fill time.
There's also always the possibility that Zunino never makes the major leagues. The team has to be ready for that possibility and not put all it's eggs in a single basket (which might also explain why they drafted a ton of catchers last year and a high profile catcher this year).
So we need bizarro world humidors!
But seriously, doesn't the ball carry better supposedly when the roof is closed? Couldn't they just keep it closed or something as a temporary band aid til they can potentially re-adjust things in the offseason?