1. It hurts female gamers
Let’s get the biggest and most obvious reason voice chat is a terrible idea out of the way: it allows easier for the harassment and degradation of female players. There have been multiple accounts first-hand by female gamers who reported that they have to deal with unwanted harassment and even stalking once they are identified on voice chat as female. A 2013 study by the University of Ohio found that “[o]n average, the female voice received three times as many negative comments as the male voice or no voice. In addition, the female voice received more queries and more messages from other gamers than the male voice or no voice.”
This problem is essentially erased when using simple text communication, like League of Legends does. Unless the player has an extremely feminine name, it’s unlikely any player can deduce any of their teammates’ genders.
Riot recently faced huge controversy regarding their culture and treatment of female employees so it would likely be in their best interests to make the game be as inclusive as possible. Riot has also previously noted studies voice chat in video games disproportionately hurts female gamers, so it’s very unlikely that their sensitivity to this issue will change.