It's a different institution. Barcelona can change the club's management via the fans like they have done several times. Also, they can "reset" if the coach can go back instilling Positional Play, which is in the DNA of the club. Since Johan Cruijff's advent into Barcelona, there has been a cycle of Nunismo and Cruijffismo. Sandro Rosell and Bartomeu are Anti-Cruijffistas and wanted the club to go with big powerful players and believed that playing direct football was the way to go. They do not care about the club's philosophy and actively tried to destroy it and remove Cruijff from the club's legacy. Then you have people like Rijkaard with Henk Ten Cate, Guardiola and Tito Vilanova and then now Xavi who follow and ad on to the model that Cruijff conceptualised.
Manchester United fans cannot change the Glazers, they own the club and answer to no one but share holders. Unlike Barcelona who have an overriding philosophy of Positional Play that runs throughout the quarry, Manchester United have no overriding philosophy. Under Sir Alex Ferguson, you changed your tactics and strategy according to the opposition and never went onto the pitch to dominate the opposition in every game the way Barcelona do. Xavi signed players to add on to an existing framework. Manchester United have no framework to add on to and the players there do not fit with each other.
Manchester City literally hired Barcelona's staff to instil a philosophy in the club to attract Pep, Liverpool used analytics and married it with Jurgen Klopp's methodology, Chelsea have a philosophy they have and it has worked for them. Arsenal seem to be on the right path with Arteta. Manchester United have no anchoring and are loyal to nothing. Louis van Gaal could have been that for you the way he kick started Bayern Munich into modern football but the club had no patience with him.
A coach like Conte would have been perfect for United as he is modern coach, but he did not fit whatever model the club supposedly followed.