It's laziness from where I stand. They decided to not bring back the people who played the guardian for each gender (maybe race, not sure if that differs) and just gave all the exposition to Ghost. It irritated me to no end seeing the guardian have no voice after the initial story, cause what is the bloody point of them having a voice at all if they are just going to take it after the Vanilla content is done? But again as far as I see it, it all boils down to simple laziness from the devs cause they provide no rhyme or reason for why the guardian suddenly became mute after the initial story. It's stupid really