I think it's the nature of the defeats with United, more than anything else. I don't recall us getting turned over as much as United have been under ETH, or at times under Ole for example. We'd lose or fail to win a lot, but wouldn't get turned over often. You've won more than we did back then.
I wonder if you'd finished 5th in the previous two seasons playing nice football but not won anything (looking like you were building towards something), your fan base would be generally happier than you are now.
We won next to nothing in the 90's but put some nice football and some decent players (Owen, Fowler, McManaman etc.) in front of The Kop and it was fine. We finished 8th, 4th and 4th under Klopp winning nothing, but we could all see it was moving in the right direction. United haven't had that for an extended period under any post-Fergie manager so far. Maybe Ole and Mourinho at some point but it turned sour for them pretty fast.
You haven't had a solid period of development under one manager like Klopp at Liverpool, or Arteta at Arsenal. Maybe you don't win anything but you like what you're seeing on the pitch, that kind of thing. Maybe that's what most of your fan base really want.