Because he has a history of losing his runner, tracking players, or however you want to put it.
It is also confounded by the fact that I think that Neville is far more assured in this position, and protects our defence to a greater degree, and from the fact that I think Fellaini is a monster in attack, and that ACM is the position where we will get the most out of Fellaini.
And the trio of Neville, Gibson and Fellaini is brilliant IMO, I don't see why we should change it. Unless it is against Stoke, who are the exception IMO, but even against Sunderland/Fulham etc, it worked wonders, the opposition didn;t have a sniff and they just cannot handle Fellaini in an advanced position.
Just my opinion, but I do believe that it is one that is shared by David Moyes.