In brief, that's all down to Ole's coaching and management. He simply acted too naive or optimistic but did not think through those tactics details nor build the system robust enough to allow his misjudgments .
Some posts moaning about our midfield squad options, but clearly the original plan has been set up to play 4231 , and Pogba / Fred / McT / Matic are competing for 2 positions . You can't say we don't have the squad ready as that was what Ole wanted. If Ole thought about a plan B to play 433 , then he probably would ask one more MC in the team . Some may argue Ole considered VDB as a MC option but he flopped, then Ole simply did not trust his squad or assess his player well enough as VDB rarely played. If Ole made a proper assessment, he should have determined that VDB would not fit , then in Summer window he asked for a new MC and let VDB go.
Another squad mystery was bringing back Ronaldo and now blamed all failure on him. Don't forget Ole mentioned couple times that he had the veto power to the recruitment, so he could say No if he did not think Ronaldo could fit in. In Ole's mind, he naively thought Ronaldo could elevate the team taken his world-class goal scoring records. But in reality, Bruno was playing the role as what Ronaldo does right now , Ole did not clarify their roles but let them play at instinct , which caused the imbalance and left midfield wide open. Ole either should see the imbalance and clarify/fine-tune their roles , or drop one of them . But he never had the gut or insight to do it.
Last season Ole stretched too thin on those key players like Bruno, Rashford, Maguire , and Shaw, they never got properly rotated and rested; and this season they are just both mentally and physically fatigue and could not perform the same level. Again Ole did not understand such way was not sustainable in modern football and they were not Lampard who can play consistent and start every single game as well. On the other hand, Ole's tactics relied heavily on individual brilliance, without these key players carrying the team, the performance just dropped drastically.
In sum, Ole did not have the technical acumen nor the strong will for this job. Of course, our board also did not have the acumen to know about it, and therefore they let the bubble flow for 3 years until it bursts.