WHY SPIRIT BOMB DOESN'T WORK IN SUPER SAIYAN FORM?

Spirit Bomb the ultimate attack of Goku but as the matter of fact, it only worked against Majin Buu. Even being an ultimate attack only killed Majin Buu, I have said the same thing in my blog DRAGON BALL Z FACTS THAT WILL SURPRISE YOU. 

King Kai taught Goku in Dragon ball Z to fight against Nappa and Vegeta. King Kai also taught Kaio-ken at the time he taught Spirit Bomb. However, the attack has one glaring drawback: it's extremely difficult for Goku to maintain and control while using any of his Super Saiyan transformations. Why is it difficult for Goku to use it while being in super saiyan form? I suppose it impossible for Goku to gather huge amount of energy in super saiyan form. 

We know only a pure hearted can wield spirit bomb only due to its divine nature. This means that fighters like Gohan were able to redirect it during the final battle against Vegeta but also limited Goku and the others when they achieved the Super Saiyan form shortly thereafter. As Goku explains to Gohan during their preparation for the Cell Games, Super Saiyan transformations require an immense amount of anger to fuel the change. Maintaining this sense of anger likely shades users' mental state, costing them the serenity required to handle the Spirit Bomb. This was visibly displayed during the non-canon movie Super Android 13 as Goku and the Z Fighters faced the eponymous killer robot in the arctic. As the battle grew desperate, Goku quickly gathered energy for a Spirit Bomb, transforming into a Super Saiyan as it neared completion. The change in Goku's personality and mental state caused the collected energy to dissipate, though it turns out this was part of a strategy on Goku's part to absorb the energy and give himself a boost. 

See also: 15 THINGS ABOUT ULTRA INSTINCT YOU DIDN'T KNOW

After gathering energy for spirit bomb he can transform into super saiyan and even higher form. But cannot gather spirit bomb in such forms. In Form of  Super saiyan God and Ultra Instinct, Goku can gather spirit bomb, as they are divine power and the power of god. Given the heightened anger required to transform into more powerful Super Saiyan forms, it is unlikely that Goku at Super Saiyan 2 through 4 could control a Spirit Bomb. More interestingly, the serene state that comes with the base Super Saiyan God and Ultra Instinct transformations opens up the possibility that Goku could potentially control a Spirit Bomb, given their shared divine origins, though this remains unconfirmed. As the most elusive attack in Dragon Ball Z, the Spirit Bomb can only be wielded by a handful of users and, for all the upgrades that the franchise's Super Saiyans gain, it comes at the cost of being able to control the devastating technique.

See also:  10 THINGS ABOUT SAIYAN & 10 THINGS ABOUT SAIYAN (PART 2)

So spirit bomb's user shall have low power level or divine power to use it. Maybe when it was invented Mortals were to weak, so to make sure mortals could also use it they made spirit bomb could only used in low power levels. I guess it shall be the history of spirit bomb.