Because although Noble 6 is a character with his/her own speaking voice, Noble 6 is still supposed to be a kind of faceless hero that the player is supposed to identify themselves with. Hence, the customizable armor and having available the choice of gender. So with that reasoning in mind, they have Noble Six speak as little as they can to retain his believability or inclusion in the team and to the overall campaign/story while also keeping his personality and image as subtle and low-key as possible so the player can project their own identity on the character and thus making Noble 6 not only relatable to everybody but essentially being every player/gamer out there who picks up Reach. That is why she/he never talks independently. And/Or if you want a canonical version of the answer as to why he never speaks so much then all you need to do is listen to what Carter tells Noble 6 in the beginning "I know you are used to working alone, but we are a team. So leave that Lone Wolf stuff behind." Noble 6 has always worked solo in the past and on top of that fact he is a Spartan, regardless of being a II or a model III he still has been trained since childhood and augmented with chemicals and hormones that make him different from most normal people. Spartans were known among UNSC personel to keep to themselves and being hard to become personable with. Being a Spartan AND a lone solo operator on numerous missions, ops and assassinations, Noble Six surely adopted a habit of keeping quiet in the field, wether he was supposed to be or not. That trait probably stayed with him through his missions up and to the fall of Reach. So....There ya go.