Because we've exceeded all expectations. The only reason people feel disapointed is because we've tapered off after the Carling Cup final. If we went to the start of the season, and made a poll saying that will you take finishing Top4, winning the League cup and reaching the FA cup final... I assure you 99% would say yes.
The reason people think this season is a disaster is because the expectations are based on our current form, not the start of season form/squad. The fact that ETH managed to do what he did so far, is pretty impressive considering where we were just 12 months ago.
As far as previous managers go: LVG inherited SAF's title winning squad strengthened with Mata. Mourinho receieved a possession based team, and did a typical Mourinho thing, brought in "his players".
The reason I mentioned Ole and not the other 2, is that even in our lowest of the low there was still discipline and the notion that we are a football club. Under Ole, all that went to hell replaced with good vibes, favoritism and worst of all: trying to appease the players and egos who were fastly going out of control.
So far Ten Hag undid all the damage did in the first half of the season. Provided he is backed in the summer, I expect us to go up a level. Can he do better? With the current resources he has, I don't think so. I geniunely think that our squad is gassed as of right now, and we will slowly limp over the finish line.