People expected way to much out of it.
Over the years the hype has gotten to the point that people were saying it's basically a PS4 game or it's exactly like real life, to much hype and expectations made for a lot of disappointed people.
That and I think a lot of the reviews were rushed, and oddly enough a lot of the review scores didn't make sense? Some reviews would be talking about how excellent the game is in whatever category and then all of sudden they give it a low rating?
Like I think it was what IGN said, GT5 is a 10/10 racer in a 5/10 game? huh?