I'm talking behind the scenes though, not the immediate results on the field. Edu came in and obviously wasn't happy with the fundamentals that he saw from Emery, so he fired him and bought in Arteta. He then liked what he saw from Arteta and gave him more time. Wilcox (and the others) on the flip side have come in to Utd and, if they do sack ETH, obviously haven't been happy with the fundamentals. If that's the case, why would they keep him?
Arteta came into a team that hadn't finished in the top four for three seasons and had currently only won 5 of their first 16 league games. He proceeded to improve season on season, ultimately challenging for the title in his third and fourth full seasons.
ETH came into a team that had finished in the top three in two of their previous three seasons. The fact that he did better than Arteta's first full season is to be expected. However the trajectory couldn't be more different, with us not only getting worse in his second season, but more importantly getting worse the longer the season wore on despite the fact he was getting more time to train the players and getting many of his injured players back for a couple of months (before many of them got injured again).