Zelda and voice acting just don't mix for reasons both internal and external to Nintendo.
As insanolord mentioned, this is just an artistic decision for Nintendo. It helps that it's economical too, but Zelda games actually have quite long voice credits these days. The fact that they are limited to "exclamations" and "flavor sounds" is purely a creative decision.
Another reason is that Zelda actually has quite a bad history with voice acting. I mean just go listen to the CD-I stuff. That would be enough to turn anybody involved with the creation of future Zelda games away from voice acting forever.
Lastly, is that this is Zelda, one of the most revered game series in the history of games. The bar is set so high that it could not feasibly be passed in one try, and if they fail badly, fans won't suffer their repeated attempts to get it right. For an example of this, look at the industrial-strength bitching about the new Metroid's voice acting, which is actually pretty average to above-average as far as games go, but because it's a long revered Nintendo series, anything less than perfection is a stab in the heart. Hell, look at how many people still kvetch about Mario's voice, 14 years later.
I also second insanolord's immense praise for the Treehouse. Considering the amazing amount of variety in games they localize, that they can find ways to differentiate by tone, period, mood, and age level is truly magnificent and they are definitely not appreciated enough for it.