The answer is simply because of Alex Ferguson. I could leave this article there but i want to explain why the manager is of such quality. Alex Fergusonn has the ability to spot players from a young age sign them up and make them into great players.
For example Ronaldo, Nani, Anderson, Fabio, Rafael, Gibson, Evans, Vidic. These players are part of an outstanding empire built by Alex Ferguson and his staff. Man Utd carry a team a class apart from any other top four club in the premier league.
Although Tevez may be on his way to Real Madrid in my opinion you can only move to a club which is a step down compared to Manchester Utd. A team ushered by golden oldies such as Neville, Scholes and Giggs help the youngsters in their quest for brilliance.
Alex Ferguson has his presence felt wherever he goes moving from top club to top club he always seems to bring away positives from every game even if his team has lost (very rare).
For me the only way Alex Ferguson will ever leave the Man Utd managers job is in a cardboard box. The only person in my mind who has the quality to be able to replace him is the Chosen One Jose Mourinho. Or Martin O'Niell i dont know why he seems to fit the job description but he just does?
Man Utd also have a good core of english players, Ferdinand, Neville, Rooney, Scholes, Carrick etc so Alex Ferguson mantains the old fashioned ways. They seem to be the ways that work, im not going to go into the inns and outs of his tactics they work.
Can anyone stop Man Utd?