Why no voice chat? I'd assume because smash is a game that's going to be played by a wide variety of people, and Nintendo doesn't want to take flak from little kids being taunted by guys telling them they're gonna rape them in their sleep. Especially in a game where you can play Mario!
I've never had a good experience with voice chat in a fighting game, anyway. Usually it's the other person whining and bitching when I'm beating them, and nasty, nasty trash talk when I'm losing.