I love all 3 of those films but... One, you have to look at Forrest Gump from a POV of movie goers at the time. Nobody had seen anything like that, it may not look like much but the special effects got a near Star Wars level reaction of awe from the audience.

Even if they'd decided against Forrest Gump, Shawshank is the highest rated film on IMDB (Rotten Tomatoes maybe?) if I'm not mistaken. So it's hard to see how this snub was greater. It's a Hollywood popularity contest at the end of the day; I don't think many people appreciated Quinten's approach, in fact they seemed to resent it as sub par cinema.