Could be that voice chat was an 11th hour addition and couldn't be added into races because those system resources were already devoted to MKTV or something.
Just a guess but... between that, arenaless battle mode and the awkward character roster it seems pretty likely that the end of development for MK8 was super rushed.
Weird to take 2+ years to rush a game, but then it's Nintendo and the first HD Mario Kart.