Howdy friends, I'm the design lead on the Behavioral Systems team for League of Legends.
Some quick thoughts:
The core goal and motivation people think about when they ask for broader voice support in League is better team cohesion by increasing communication.
I actually think that's a noble goal and if that were the only thing at play I'd be super gung ho about team voice chat.
Unfortunately what we've seen time and time again is that the more rich the communication tool, the more powerful it is as a means for disruptive behavior. Put more simply: It's way worse to be told you're a noob via voice than it is to be told the same through text.
And beyond that, voice is more damaging and disruptive to marginalized groups. It reveals more information about them that has been repeatedly demonstrated to increase severity and frequency of harassment. (Think gender, accent, dialect, etc)
And to go even further beyond that, voice chat is significantly more difficult to moderate, detect disruption in, and police than text chat or other forms of communication.
In the future, we will certainly be looking into giving players richer communication tools that do not have the same disruptive potential as voice chat. For now, we like voice chat for parties and don't love it for teams.