Stop saying other games like cs:go/overwatch have voice chat, those games are absolutely DIFFERENT from League. In cs:go you have 30 rounds, those rounds are much more dynamic than is one game of League, you see someone coming long, and you don`t have time to write in chat “they are coming long”, neither you have option to ping it like in league, just stop comparing moba to fps games.
Another reason is Riot will no longer be able to ban people, because it`s impossible to do it via voice chat logs.
Next reason is maybe something you wouldnt want to hear, but voice chat is not necessary for your silver games, if you are not playing 5v5 tournaments, you DONT need voice chat, all you need is to ping, and maybe write “go baron”. Simply put, dont think you will magically get higher elo because you can communicate via voice chat, its much more of a distraction to many people, rather focus on your own game, its solo que after all. You have enough to do as it is: map awareness, trading with your enemy, csing, you dont wanna another distraction like talking to 4 strangers and listening to them at the same time.