Seth Enosh
Member
Smash Bros Fact 49 from the 50-Fact Direct says "while you're battling, in order to keep your connection as strong as possible we've disabled voice chat". Not sure if they ever gave a reason for Mario Kart 8 customs but it could be the same.Regarding Mario Kart 8 and Smash Bros, which reasons did Nintendo give regarding voice chat there? Those games have voice chat, although not in-game, but you can potentially do shit talk when voice chat is available.
Based on what the director of Splatoon said, not having voice chat was his own decision and not a company policy so the decisions made for MK8 and smash may not be relevant. It will be weird if the 8 player custom lobby mode released in August doesn't have text or voice chat though.