A range of factors..
1) Dan James out, Ronaldo and Sancho in has changed dynamics. Rashford out for a long time with injuries.
The quality isn't the issue here, its the setup. What we had before needs adapting. See 2)
2) The very limited kick and run counters worked more often and longer than they should and our changes have meant we both can't do it as well as we were before and teams have learnt to counter. We need a better way to play, more technicality, more range of options.
3) Unstable defence....Varane injured more than not has killed stability. That mixed in with Maguires injury and form issues.
McTominay and Fred have also been injured/ off the boil.
I'm sure part of the answer here is being over played for a long period of time in a post covid catch up so bodies are probably not as finely tuned as they were.
4) Pogba....we've constantly been trying to shoe horn him in and the dynamics just don't work. On left wing he has been decent, but again see point 1 we lose the counter attacking speed of Rashford.
5) Momentum. We've had the toughest run of games i can remember going back to SAF days and we've gone into this run on bad form.
There is a reason I asked the question on one my threads about will Ole last till December. Frankly any manager in his position would have struggled and a bad form start can happen to any manager which we simply couldn't afford going into that run.
6) Discontent...Mata new deal no play, Jesse staying...no play. VdB signing...no play. Same as Telles, Dalot, Diallo, Cavani since Ronaldo. Many of these players have not had enough chances, have shined when given chances or have not had enough momentum to progress. This has to build frustration.
7) The biggest problem and linked to most of the above is our midfield is one of, if not they poorest midfields in the Prem. The kick and run tactics got us by over compensating with those tactics, but without the over compensation of extreme speed of James and Rashford on form you actually need to play ball.
Our midfield are a disabler not an enabler of all the quality we have. I can't see any manager dramatically changing things without fixing that. The answer might be getting creative with youth or something crazy like Cavani in midfield, but something has to change.
This last reason is also ultimately a big factor in finally losing patience with Ole. We had all the tools, we just needed to swap some. Sell some buy others. Sell Jesse, sell Pogba, release Mata, etc and reinvest in an engine. Tweaks like this would have dramatically improved the balance of the squad at next to no impact on quality of those positions.
This last point may be on the Board and might be why Ole got more time than he should have done.
Ole was definitely part of the problem and probably quite a big part, but he is far from all of it.