WoW is a completely different gaming audience. Do some users, especially raiding guilds use voice chat? Of course. It’s been that way almost forever. However, WoW, unlike Overwatch, is not a game entirely built around team game play. For many players, that is not their primary focus and even when they do partake in the occasional team game play, probably either don’t see the need to, don’t want to, or aren’t comfortable with using voice chat.
Which begs the question. If there are already available better options, why does Blizzard feel the need to spend time and resources putting this into the game? There’s no great need for it and they’re already fighting an uphill battle for adoption.