Link speaking would cause a lot of problems for everyone...
1. Would take longer to develop the game. time costs money, which would make it more expensive (durr), and I want this game as soon as possible, thanks. They'd need to synch the speech with the graphics and if it was off even slightly I'd go apeshit.
2. We all imagine him differently in our minds, and I know that if I got lucky and heard exactly the voice I'd been imagining, someone else out there would be disappointed with what they heard.
3. Being silent adds to the whole "non-real language" thing - Hylian. If he spoke Japanese or English it would detract from the magic of his foreign-ness.