Why can't Goku use Kai Kai?

I'm not sure if Goku can since Kai-Kai is a technique used by the Supreme Kais (and apprentices) exclusively. But, since Goku learned the Hakai technique (in the manga)… it's quite possible he can learn Kai-Kai. read more

I’m going to assume you meant Grand Kai /Dai Kaiou, because Supreme Kai is an entirely different Kai, he’s the Kai who Elder Kai tricked into doing a Portara fusion with Kibito, where as the Grand Kai was the Kai who held the tournament. He promised Goku and Pikkon to train them in 200–300 years, that’s because he was much weaker and out of shape compared to them both due to neglecting his trainings. read more

The Supreme Kai was no match for Buu. Goku was. And that was before he reached the level of Super Saiyan God, not to mention super saiyan blue. Goku is leagues beyond the supreme Kai. read more

SSJ Goku using a simple Kaioken x2 would put Goku on what Super Exciting Guides consider a Super Saiyan 2, and Kaioken x8 (well he skipped to 10) would put him on the level of Super Saiyan 3. It's largely considered that Goku can't handle that massive power up in his body in such a short time. read more