Presumably because of the - conveniently non-quantifiable - factor called "Moyes destroyed them mentally". In the most extreme (and not entirely uncommon) version of this fairytale Moyes "inherited a squad of champions", then proceeded not only to do a historically poor job but also to break these players down psychologically to the point where they were practically useless - to LVG, who then had to rebuild their shattered psyches in addition to everything else.
The more reasonable explanation is the one presented by
@Speak above.
Your point is fair. Speak's opinion is also fair enough. But when SAF resigned, the club probably did take a bit of risk in hiring Moyes. But if Speak's point is valid then probably club would (should) have understood the situation Moyes is under and the fair amount of blame would have gone to the players than the manager itself, right? (The naive thought would be club overestimated the squad and we simply didn't have beyond the doors personality able to access the squad but Woodward is a businessman and so are Glazers, Man United is an asset. They simply won't be overly stupid in that regard in assessing their asset.) If that was the case, then what was the point of bringing in a new manager and an entirely another stage of evolution rather than giving Moyes the trust for another season when he had assessed the squad well for almost a season? While our club doesn't have director of football and others, we still have fair bit of ex-players and managers in director's role, who could have quite easily pointed out the flaw was in the squad and then we might have stuck with him for may be a longer time. I don't disagree with the decision to get rid of manager but if the blame is to principally in the squad, then probably Moyes was harshly judged. LvG made a wholesale changes and played a style where we ride our luck fair few times when winning games 2-1 away (Arsenal, Soton) and still managed to finish only 4th which is not a great position for a club of our stature and finances. This puts LvG's performance in a bad light though.
It's almost universally accepted that Moyes didn't have the ability to get the best out of this squad and underachieved, which if he did puts a lot less blame on the players. I don't think psychological factor is huge. Liverpool managed to make the jump from 7th to almost winning under the same manager and almost same set of players. We can't simply say our players were mentally knackered just because of one bad season. It invalidates the point that player are fairly confident of their ability and back themselves of their ability and even perform better when under pressure.
Probably the fair standing is somewhere in middle, the squad was piss poor and key players were in decline and Moyes didn't have correct mentality and authority to take club ahead in right direction in long term. Disregarding all the factors said (big players want to play for LvG not Moyes, Moyes didn't have the balls to clear the squad), we have probably been harsh on Moyes then while praising LvG for doing something which is fairly expected. After all money brings success over time and we are simply too rich not to be challenging for titles in few years time. ( I read the last point in Rawk of all places, so its probably universally accepted).
. Your insight may have been spot on but I don't think he had many good performances that season, and when he did it was as a CB. Hence why he was so unpopular. You were one of the few who could see past that. I still can't believe the amount of abuse I got in one post where I labelled him as our best defender in terms of actually defending.