User blog comment:KidVegeta/Canonicity/@comment-3097771-20101130082822/@comment-3097771-20101130085708

By the time of Android 19, Goku had trained in Super Saiyan for three years. Hardly the same as when he faced Cooler. His Super Saiyan form at that time was still unrefined and unpredictable, and thus I don't think he could transform, immediately, while in the middle of fighting. -KidVegeta