I don't have the minute count for the film but the rules require:
An animated feature is defined by the academy as a film with a running time of more than 40 minutes in which characters' performances are created using a frame-by-frame technique, a significant number of the major characters are animated, and animation figures in no less than 75 percent of the running time.
It's possible that the live-action sections added up up to over 25% of the film's content, and caused it to be out of the running.
That being said, I have a difficult time believing, from my memory, that the live-action portion was 25% of the film.
As noted by Ankit Sharma in the comments, my memory is correct. The film was 100 minutes and the live action section was only 10 minutes long, making it only 10% of the running time.
This means that this technical reason is not a possible explanation for why the film wasn't nominated.