To echo many others here, Generations is being overshadowed by many other games. Had this game released on the actual 20th anniversary in June it might've got a bit more acclaim, but right at the height of Triple A release season it was always going to be overshadowed.
Furthermore, Generations is the game the fandom wanted. It's a re-imagining of the best bits of the main console games from the last two decades. Which is great for the fandom, they'll lap it up instantly, along with all the in-jokes, references, unlockable music and art as well as the big massive collectors edition statue, CD, DVD, shiny box set. But that doesn't really appeal to those outside the fandom.
Platformers are, unfortunately, no longer the popular genre. Back in the days of Sonic's origin's in the early nineties Platformers mattered. The consoles of the time couldn't hack hyper-realism, or even the third dimension. Hence the trend at the time was platformers, because the software could not only handle it, but make it look limitless. Shooters were still mixed with platformers, with bullets that only fired in eight directions, which killed the realism somewhat.
Lets also not forget that at the time the target market was kids. Teenagers and adults (the target market for pretty much all the games that are overshadowing Generations) weren't overly considered. Hence a great looking, quick platformer about a cartoon hedgehog was the biggest selling game on a system. In addition, certainly after 92/93, Sonic wasn't just a videogame. There were two cartoons, and a comic (well, two, but as a kid i doubt you knew what was going on the other side of the pond). Sonic was really popular, the big playground craze. You put Sonic on something in the early ninties and it would sell on the star power today, star power he no longer has due to a decline in reputation, the fad ending, and that generation growing up.
Finally, Sonic is Sega's mascot. He is the face of the company. When Sega was a hardware manufacturer, a new Sonic game was a big deal, so much so that Sonic 3D was remade for the Saturn to give the console a Sonic game, as Sonic Xtreme fell through. Not launching with a Sonic game would be like Nintendo launching a console and then never making a Mario or Zelda for it. But since Sega is now just a third party publisher, Sonic titles are less relevant in the overall scope of gaming.
Tl;dr Sonic is no longer as important as he once was due to Sega's actions, and due to the original audience for the game growing up and wanting games about dragon hunting/realistic shooters, hence Generations is overlooked by all but the hardcore Sonic fanbase.
