Manchester United club losing its winning mentality should be one of the reasons, Manchester United is yet to sack Ten Hag. They don't want to make the mistake of sacking Ten Hag and the same mistake is still there, affecting the team to win matches.
About mentality in Manchester United, do you think it is coincidence that Mourinho said "Ending Premier League as a runner-up is one of his biggest achievements in his coach career"?
This statement from Mourinho, years ago, reveals that something were very improperly in Old Trafford, not only from their coaches, staffs, players but also from people at higher positions.
Manchester United taking their time to make a final decision of sacking Ten Hag or not is a good move. At least let it be noted that Ten Hag was given enough time before they sacked him. It will now be Ten Hag's duty to return the Red Devil to its glorious days, now he has the chance to prove to the whole world that he's qualified to continue coaching Manchester United. If failed to, he should blame himself for that
These problems continue to exist through many coaches and even with Ten Hag, they are still existing and effects are harmful for the club performance, results in many competitions generally.
I see it is very unfair to say Erik Ten Hag is responsible for all bad things that are happening in Manchester United. He must be given more time like how Liverpool were patiently with Klopp years ago, then they harvested sweet achievements with one Premier League, one Champions League trophies. Manchester United can gain a same with Ten Hag but they must give him more time and support him to have players he wants.