Why do they yank Sogard, Hamilton, Eaton and others from AAA to BOS and back? Because they ultimately don't believe they have longterm value for the team. The see these guys as fungible assets that they can plug in as needed. They've left Campbell in AAA because they care about his progress and don't want to mess it up again by promoting him before he's ready. They see longterm value in Campbell and are going to protect that asset. Campbell looked awful in ST and probably didn't deserve the Opening Day spot. They trusted him and let him run with it anyway. Ultimately, it didn't work out.
Campbell has not made any adjustments this season. He is still struggling. He's whiffing at pitches in the zone and not making hard contact or pulling the ball. His defense is also not getting any better.
Anthony is the far superior prospect no matter what award Campbell won last season. Campbell had his shot this season and saw his numbers plummet. Anthony had his shot this season and had his numbers climb throughout the year. When pitchers made adjustments to Anthony, he kept hitting. You can't say the same for Campbell.
The fanbase isn't down on Campbell. They are being patient with Campbell and are honest about his true ceiling. He should be a fulltime player at some point, but maybe it just shouldn't be this year. That's how it works with prospects sometimes.