You are forgetting this is an international server. While most can chat in English, speaking is a whole different thing. “Well, then just listen”, you say? That’s an even bigger problem to those who are not very fluent yet. English is not just English. There are myriads of accents, from US to UK to random EU people, each very hard to understand unless used to. I’m pretty much fluent in English, and am able to speak it as well, but listening to native mumbling nearly drowned out by noise from their surrounding is hell to me still.
The problem with introducing voice chat is that certain people will come to expect you to at least listen in. It’s completely different from having a guild chat via an external application where you have the choice of whether join one that uses voice chat or not. While I don’t think it will get as far as @Kirosuu said, there will be certain people that will get annoyed at anyone who doesn’t use voice chat (aka “I just TOLD you, why aren’t you listening!”). It just adds additional stress to gameplay, and stress isn’t what you usually look for in a game.