nah its simple.
-higher salary (had 4.5M here and we would have given him 7.5M, at United he has around 9M or more)
-the club he likes ("home", "family", "return from a holiday")
-the club is actually top of the world in terms of marketing and making money (wants to build his own brand)
-the club is the biggest in a league thats top of the world in marketing and money (see above)
sadly, football is only secondary to him and establishing himself as a brand is his goal. and there is no better place than United for that