I can't speak for anybody else, but I feel that the animation community, particularly the Hollywood-based groups and individuals that discuss animation history and such, are so biased towards anything PIXAR and Aardman does that they tend to neglect everything else out there. Do both studios produce quality work? Absolutely. But that's not to say other studios aren't. Sure, they'll acknowledge them with nominations, but the general consensus, especially to those that actually pay attention, is that if a PIXAR or an Aardman product gets nominated for something, one of them is going to win.
It's like they're automatically destined to win just because they are from those studios. I'm kind of sick of that arrogance myself from companies, individuals, politicians, groups, and sports teams.
When Happy Feet won for Best Animated Feature at last year's Oscars, many were shocked. Not because Happy Feet won, but rather Cars lost. I personally felt that Corpse Bride was a much better film than Curse of the Wererabbit, and I wasn't alone in that wave of thought.
Noticed you didn't mention A Bugs Life or Cars. Heck, you didn't even mention the Toy Story movies nor Finding Nemo, the few PIXAR movies that truly deserved every praise it got. Dreamworks only shifted to 3D because of mismanagement and pressure to be like PIXAR (at least, that was the idea). I think what's damaging their credibility is the overreliance on the Shrek property, which should have been one and done, move on to something different, and focusing more on the voice talents than the actual project.
Shark Tale was abysmal, as were the Shrek sequels. But Madagascar and Over The Hedge were pretty good. Bee Movie was so-so.
People criticize Dreamworks way more than any other studio, mostly because they're trying to hard to be PIXAR in their eyes, and that's unfortunate. The thing about PIXAR movies is that they're the same story with different elements. Toys, bugs, monsters, fish, superheroes, cars, and rats have all served as the avatar for the same story. The surroundings change, but it's still the same fricking story.
Of course, a lot of stories from other studios are like that, but with PIXAR, it's so obvious. The ones that stray away from that prototypical archetype are the ones that should be honored and celebrated. And the international animation industry aren't trying to be like PIXAR. That's why you see a lot of excellent animated films and shorts from Canada, the UK, France, the Middle East, Japan, and other places. They don't get noticed here because they aren't PIXAR. A lot of those films are just as good as (if not better than) anything PIXAR brings out.
And for the record, other artists and studios are still working on stop-motion projects, but Aardman gets more attention than the ones out there.
And that's . . . unfortunate.