De jong is seeing barcelona as his home and he will never ever leave from this club to play in another club. Barcelona have made several attempts to sell him to another club, but it has proven to be a difficult task. As you said that manchester united has ever tried so hard in getting him but he was always rejecting MU's offer. He is not having intention to leave from barcelona. De jong is now received a new contract from barcelona but his salary is already reduced. It seems like that barcelona was willing to extend his contract by reducing his salary.
I don't even know what's the final decision from him but it seems like he will reject the new offer from barcelona. I remember when xavi never had intention to put him into the main squad but it surprises me to see how xavi was changing the decision regarding him.
So even if Manchester United attempted to get the player on their team he refused to accelerate the proposed contract with Man United and decided to start out with Barcelona.