Because when we sent Sir Alex to Guardiola and leaned on the storied history of the club, it also failed miserably?
Here's my take: United were lucky to have two exceptionally good Scotsmen in their history, otherwise, things look pretty unimpressive. British managers have been nowhere near the top for decades, Sir Alex aside. And northern teams have been unable to attract top continental managers and players. The best league manager of all time at a point when football was going global has given a new generation of fans a false sense of United's stature, and having grown up on unparalleled success, they don't know how to deal with it.
Liverpool struggled with that too. Shankly and Paisley were exceptional, they tried sticking local, it backfired, they tried second tier European mangers, they mostly failed. Klopp's time at Mainz set him on a unique career path that Liverpool were the perfect team in the perfect moment. When time moves on we'll see how it was a blip. The cream of the British crop wont get you to the top of Europe, and the European elite don't need to drop down to United's level.
We can go down the City route, and probably will. Until we're there, we're 2nd tier at best, usually lower. They just have better choices. We can get Pochettino but we can't get Guardiola. It is what it is.