It also comes down to position. A scrum half can always get in the ref's ear. It's harder for the wing/FB if you have to run in to talk to him every time.
Also (again I do love MB but)....he doesn't have the temperament to talk to sir. We'd end up getting marched back when the red mist descends.
I don't think that would be the case, but this reminded me of a time when MB took over the captaincy on the field when the captain went off (I can't remember if last season or the one before), something happened on the field at MB went to talk to the ref, who told him to go away as he wasn't captain, MB was quick to remind the ref that he was.