They didn't bring back voice acting because the Guardian is supposed to be you, yourself, in the game world. Remember when they added voice acting for Fallout 4? People hated that. Especially since it was one generic voice for each gender. That took away the image that you are the one who is saying something to the NPC, not someone else. Same thing with Destiny. How your character reacts to the world is how Bungie expects the player to react, not someone else. That is why some silent protagonists, like Link from The Legend of Zelda, are loved.