I used to think it's because Hugh rarely campaigns, but he actually tried the past couple of months and did campaign. Maybe it's because of his personality? Usually everyone so passionately talks about how nice a guy he is instead of his talent and acting ability. The irony is that he rarely promotes himself and he seems very humble, but when he does promote himself, he gets criticized. Many of the other actors promote themselves all the time, and when they do, they are apparently in the right because they are regarded as "better" actors.
I have no idea why so many movie fans forget about him too though. Usually, I see people arguing on other forums that, for example, Christian Bale was way better than Hugh in The Prestige, Jake Gyllenhaal was better than Hugh in Prisoners, etc. etc. I'm usually in the minority on forums when trying to give Hugh credit.