Why? The reason he plays up front for Spain and for Barcelona is the same. They've a certain way of playing and there are players better than him in that role. Considering the talent of those players, it's not a bad reflection on Fabregas's potential at all. The fact that he hasn't set the world alight there also means that is not his best position. What we do know is he was one of world's best players at CM before he left for Barcelona. Since the day he started playing for the first team uptil he was 23, he was a CM. Why would 4 years of indecisive positioning change that?
And I personally think that provided we don't play a conventional 442, he would be a brilliant signing for United as well.