Academy was too snobbish to nominate Lego and some idiot voter went on record saying "Who cares about this Chinese thing?" regarding Kaguya (and Song of the Sea which wasn't even an Asian production but racism follows no logic). So it was just between BH6 and HTTYD2. BH6 had the edge as it had a better release date, better box office, was WDAS just coming off the Frozen phenomenon, and it was the first film to heavily feature the Hyperion software's super rendering technology.
Also, as far as the general public and most Academy voters were concerned, Big Hero 6 was pretty much an original property thanks to how obscure its source material was and how completely cut off the movie was from other Marvel properties. Teen Titans Go! and Into the Spider-Verse have a more uphill battle as they feature more marquee characters.