Stacked up against the other nominees, Pixar just didn’t bring enough to the table. Inside Out was decent but nothing spectacular. I would have actually preferred The Good Dinosaur to have gotten the nomination this year.
But of course, the voters don’t care about the animation category so they just picked whatever was most popular/whatever their grandchildren liked the best.
Fact is, and I hate to say this, Pixar has slowly been declining in quality over the years. John Lasseter always said they’d never do sequels unless there was a good reason and they’d found the right story. Now we live in a post-Cars 2 world and the future isn’t looking that bright.