User blog comment:KidVegeta/Canonicity/@comment-3097771-20101130082822/@comment-3097771-20101130083417

It is unknown if he just didn't go super saiyan, because he thought he could beat Cooler. That's what I'm leaning toward (it's the most logical explanation). Also, a majority of Dragon Ball media consider this movie canon (while also not considering others canon), so I think others agree. -KidVegeta