As far as I'm aware it's simply a technical issue. The amount of data the Wii can transfer isn't much compared to other systems, and good-quality voice chat uses a lot of data. This is why data-heavy games like The Conduit and Monster Hunter Tri have such poor voice quality, whereas a simple game like Animal Crossing works fine. So, getting a game to work with voice chat is quite a challenge, because it requires getting the game itself to run using even less resources than what's already available.
Now, text chat, on the other hand, is extremely simple to implement. That's the one where its exclusion is more likely due to being weary of offensive people.