2 reasons.
1. Bardock Father of Goku was not created by Akira. He did like the character of Bardock and decided to make him canon but the story itself was never ever stated to be canon. So until it is classified as canon by Akira himself, or referenced in a canon storyline, then it and all spin-offs(including Episode of Bardock) will be considered non canon.
2. (And this is the better reason since it can't be argued against) Akira recently wrote a canon story about Bardock and Goku's mom while they were living on Planet Vegeta. That story itself contradicts almost everything in the original Bardock movie. That cements the film and any spin-offs as non canon.