He is a very good goal scorer but not that good at anything else.
Unlike Aguero or Tevez for example who are both good finishers but also can dribble, pass, create and generally do more than just stick it in the net.
If you watch all of his goals for us you will see a good portion come from when we aren't winning. This means that we end up going a lot more direct and generally just look to get it in the box asap which suits him.
With Aguero and Tevez in the team we just won't start games like that and will always use it as a backup plan.
He also isn't good enough or he at least didn't show that he was good enough to play as the traditional target man consistently enough and have the likes of Aguero, Tevez and Silva etc playing off of him.
The rumors could be rubbish and people are betting on nothing. Perhaps under Pellegrini he may find a new lease of life, especially with having an out and out winger giving him the service he could end up being a very important player in the team.