The difference is Sancho cost about £8m, has gone to a different country and put in the effort and racked up the appearances, goals and assists. He looks like he wants to be a professional foitballer, to improve and play for a big team and win trophies. Whether he does that at Utd or not we'll see if he joins.
Dembele doesn't show any of that, cost Barca €135m and he has absolutely stunk up the place with his rubbish attitude, terrible injury record and generally been seen as lazy and unprofessional. Eats junk food, moans about pretty much everything and doesn't even show up for training on time and sometimes not at all. Deschamps has even said similar about it with France. There were even reports that his own team mates didn't like him, Rakitic said they were playing with ten men one game he played.
Why anyone would want that at the club is beyond me.