Here's the key difference though: go to any community centered around this game. Miiverse, here, etc. Count the number of people who say things like "I carried my team and we still lost!" constantly. Those people probably aren't interested in actually working with their teammates in solo queue, but rather in taking charge and barking orders to everyone. Now, is that an attitude that every player has? No! But it's definitely a pervasive mindset, especially among the people who play the ranked modes. Now, imagine putting even two people like that on a team. There's not going to be any coordination; only two people shouting over each other "No, do THIS thing instead!" and confusing the rest of the team. That provides no help to coordination at best, and at worst everyone's play is negatively affected from focusing on the bickering (unless everyone on the team turns off voice chat, but then what was the point of including it to begin with?).
I'm aware that scenario is exaggerated and won't always happen. But the problem is actual coordinated teamwork, no matter what, isn't going to happen with random people that the game matches you up with, due almost entirely to situations like that. It takes a certain amount of trust between the players to be able to coordinate things properly, which you won't get when you pair up random people who all want to be the head honcho. I also think the lack of voice chat helps people subconsciously get better at the game; if you can't bark your orders to everyone else, then to do well you have to learn how to make your actions complement your team's without any real influence on what they do.
That said, voice chat in squads and private matches? There's not a problem with having that. It's just not something that's going to work in solo queue.
