Blame the virus for its failure.
Someday will it be seen a forgotten Pixar gem that never got its due? Who knows.
It hasn't been particularly rated high among critics. Onwards is a middling to average film at best. Nothing original about it. If you remove the weird characters, it could be told and set in a normal world... so it's really nothing special other than Pixar's stunning ability to animate and design.
That said, this film might receive a small cult following, but overall, I think it will be a forgotten Pixar film never getting the limelight it may have deserved. It will just be forgotten... like "A Bugs' Life," "Brave," and "The Good Dinosaur."