User blog comment:KidVegeta/Canonicity/@comment-3097771-20101130082822/@comment-3097771-20101130085324

Different circumstances. It's much harder (and riskier) to transform, when caught off guard, mid fight, with a power that you have barely trained with. When he transformed for Future Trunks, he had no pressure, and didn't need to use the form. -KidVegeta