How can one speak of "unknown reasons" when on the surface there are several reasons why this transition was unprofitable for De Jong? Debts that Barcelona owe him + demotion in class (I'm a United fan, but now moving from Barcelona to United that doesn't even play in the Champions League is a demotion in class) + his girlfriend who prefers Spain.
De Jong is playing very well right now. I think the reason why he didn't leave Barca was because of his love. This could be an expression of his love for Barcelona. Also, if he goes to Manchester United at this point, it will take him a long time to adapt and there are doubts whether he can play in the first XI. These are my personal opinions.
If De Jong moved to United he would have a guaranteed place in the starting line-up - he plays well + he is promising due to his age + no one will understand buying a player for a huge salary to sit on the bench. Maybe next year, if United get a place in the Champions League, negotiations on this transfer will resume. Barcelona have a big problem with the payroll and will try to cut it down.