Because they handled it well. All teams have some big transfers that end up failing but it depends on how you handle it.
They gave him a chance for a couple years then realized their mistake and loaned him and let him free to go. They didnt extend him to keep him as an asset, they didnt haggle to get a few extra million quids just to jeopardize his departure and have an unhappy player in the locker room. They accepted the loss, didnt increase that loss by paying him longer than they should, and moved on. That's what Man Utd should have done with Maguire, Martial and all those players who are still here because they dont want to lose "assets".