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.
I agree that De Jong has love for the club and that will be the fundamental reason why he decided not to leave in the summer, he wants to play for the club but wants his wages owed to be paid, in my opinion it was a wise opinion to stay back at the club where he already build a fan base and good relationship with the players and coaching staff already. THere he feels at home and will play better.
He seems really know that going to manchester united is a big bet that his career could really in risk.
I mean if you look at the current manchester united situations where they are struggling to achieve something , their last trophy is a europa league back in 2017 which means it's been 5 years and counting! Where at barcelona he could growing still both in career and achievements.
Pretty reasonable one though.