Zelda is not a good reason to buy a Wii. Zelda is available, in its entirety, on another system that costs 1/3 as much, and is five years old so you likely already own one. You buy the system for the remote, not for the game. Hence, Zelda's not the Wii's killer app.
Well...
You were NOT able to buy Zelda:TP for any other system but Wii.
Going buy your logic... Halo was not a killer app for Xbox, because it also went to the PC. The same for Halo 2.
Was MGS 2 not a killer app for PS2? I personally played it on Xbox.
Zelda: TP is definitely a killer app on the Wii.
Sure, the time gap in those examples I listed is far off from the gap between GC & Wii Zeldas, but still...
It was a timed exclusive, and it does have some exclusive features that make it THE one to own (I couldn't imagine playing it without the Wii controller, and I certainly couldn't imagine playing it in the crappy 4:3 aspect ratio that the gamecube version is limited to(unless of course I still had a 4:3 tv as my gaming tv, but I don't so...)).
Also, as some have said, the controller is the killer app of the Wii. No, it isn't an app, but most of the "apps" that it is used for are enhanced by it, and it IS the selling point of the Wii, which is what would make a killer app killer in the first place.
With the refreshing new take on controls that the Wii offers, and its collection of above average titles that make use of that controller, the Wii didn't need to have some heavy hitting juggernaut (though I stand by my statement of Zelda:TP being that).
BTW...
If you ask my kids, the Wii does have a killer app, and that is the Wii Sports title that came with the Wii.

I do understand what you are saying about a title that pushes what the system is truly capable of, but that rarely happens at launch anymore for any system. Those kinds of titles take millions, and years, so with a launch these days, it is great just, at the very least, to have some titles that are fun, and the Wii most definitely provided that.