A disappointing game is often worse than an outright bad one. When something's terrible, either riddled with bugs and glitches or delivering a gameplay loop that fundamentally just doesn't work, it's easy to rip it to pieces and then move on. The cycle is always the same; the game comes out, the internet tears it a new one for two weeks, and then players move on to the next big release and don't give it a second thought.
Disappointing games aren't quite so lucky however. Just look at Anthem from earlier this year; not a day goes by where the game isn't blowing up headlines in the press or dominating forums, and it's not because it's outright bad, but because there's just enough good in there that makes it such a frustrating experience, because it should be better.
This is compounded even more when a disappointing release actually boasts some new, unique ideas or features. Whether the games are built entirely around these systems and mechanics or just included them as afterthoughts - with the devs not quite understanding that they had something special on their hands - they're good enough to have players pining about what they could have been, had they been included anywhere else.