A VC game would be too 'risky' for Nintendo, let-alone boring (not for Nintendo, just in general).
Nintendo wouldn't want a game in which the sole purpose is interaction with others. Seeing as a lot of young children have DS's, they could come into contact with any 'weirdo' (someone on these forums). Which doesn't suit their family-friendly image.
They could of course make it friend code only.. but then... it'd be as useless as pictochat (almost).
Personally, I wouldn't really want a voice-chat game for DS... Just use Skype, or a phone...