You reminded me about Enter the Matrix, does that count too? There was a lot of hype for that game, and when people realised you couldn't play as Neo and it was pretty buggy, it died pretty quickly, at least in my friendship circle. Not sure how it was received / remembered outside of that though.
Not only was the game forgotten, it seems like the entire Matrix Franchise was forgotten as well. Series started off with the potential to be the Star Wars of our generation and then it just completely disappeared from the cultural zeitgeist following the failure of the two sequels.
Edit:
My own contribution. I feel like a lot of people are posting games that were hyped that failed to deliver so I'll post games that were hyped, delivered in terms of quality but not sales or word of mouth buzz.
Vectorman - Was supposed to be the Donkey Kong Country for the Genesis but was released near the end of the 16 bit era when everybody was more interested in shiny 3D toys. Did well enough to see a sequel at least.
The Earthworm Jim Franchise - This is one of my favourite 16 bit platformers ever. I think the game delivered on its hype, however it's kind of unfairly forgotten because it was one of those great 16 bit games that never made the smooth transition to 32 bit.
Weaponlord - The last GamePro issue I ever bought before deciding that GamePro was too kid centric for me, featured this game on it's cover (October 1994 issue I believe). I think the timing of the game was really bad. Was scheduled for a 1995 release, the same year as the 32 bit era started and everybody wanted 3D graphics. I've never actually played the game myself, I just forgot about it.
The Quake Killers - Back in I wanna say 1997-98, if you were making a 3D FPS, you were immediately compared to Quake. FPS were dubbed Quake killers. Of the ones that people were hyping up, only Unreal and Half Life really delivered in quality and sales. Literally every other game from that era was run over by them or imploded by the development. Slave Zero, SiN, Duke Nukem Forever, Prey etc.