Voice chat is a resource hog, even in the best implementation. There's just no going around the fact that it either needs to transfer a lot of data (bandwidth hog) or convert the voice on the fly into smaller sound data (hardware resource hog). If it was mandatory then it would make the game unplayable for those that have computers just powerful enough to run the game.

Not to mention, money spent on servers for it to not break the game would be better spent on hardware to upgrade the actual game servers.