It would be tempting to say that many of the people who don't get it might be newer fans, or casual fans, but I think that would be a cop out. I'm not trying to be some Top Red or anything, but it does feel like the general atmosphere among United supporters has changed in recent years. In a way it's understandable. Until 8 years ago most of us had only ever known one Manchester United manager, and one type of Manchester United: the Fergie / Gill Manchester United. Fergie and Gill leaving at the same time was always going to be a massive cultural shift even if the board and owners were up to the job (which they 100% haven't been), as well as a massive cultural shock to many fans. We just don't know how to handle it.
We're used to essentially God sitting on his eternal throne. You might not always 100% agree with God, but he's been there all your life and will continue to be there, and you know that ultimately he's right more often than not, so you learn to accept it. It's just the way things are. But now God's throne is empty, and everything is in a state of flux. The board and owners certainly don't seem to know what the hell to do now that the massive personality that steered the whole club for near 30 years is gone. And we, as supporters, don't know how to be anymore either. We can't rely on Fergie knowing best, so we're questioning everything, and flailing about in a desperate bid to get back to the status quo we grew up in.
Most other football supporters are used to managers coming and going, but we aren't (you can kind of see the same thing happening at Arsenal). We're used to things working, and we're used to the occasional bad blip getting turned over in short order as Fergie rolls up his sleeves, reinvents himself once again, and conjures up another conquering side. But that's not going to happen this time, and not only is that a hard pill to swallow for many of us, it's somehow literally an incomprehensible idea.