Since when Beerus appeared in Dragon Ball Super, many fans including me are wondering where was Beerus when Buu was trying to kill the Supreme Kai in Dragon Ball Z. If Buu would have killed the Supreme Kai in Buu saga, Beerus would have died without even knowing. Also, Whis was looking out everyone in the Universe, why did he not wake Beerus up to the kill Buu and save the Supreme Kai and Beerus. Akira Toriyama seemed to have hardly provided any good reason about this or did he forget to connect them.
Let's reason this out.
When Buu first appeared killing other Kais that time Beerus might not be the God of Destruction and the previous God of Destruction might not be as strong as Buu. Even if Beerus was the God of Destruction that time too, the older Supreme Kai was trapped in the sword making him safe. If Buu would have found out about the sword, then Beerus might have shown up. Then Buu was trapped in the ball and the current Supreme Kai was appointed as the Supreme Kai.
In Buu saga, when Buu was killing everyone, he could have killed the Supreme Kai as well. But Beerus did not show up. The reason behind this might be the prophecy by the Oracle Fish, according to which after 39 years an equivalent warrior will show up. Some details might have skipped out in the prophecy like Beerus will be fighting the warrior named Super Saiyan God after 39 years. So this means, Beerus is safe for 39 years at least. Also, Beerus is least concerned about the death of other Kais. So he did not appear to save the Kais.
Beerus woke up early and fought up the premature equivalent warrior. 39 years have not finished yet. So he will be fighting the Super Saiyan God again in few years completing the prophecy. That is why Beerus is always concerned about his place as God of Destruction.
In short, Beerus not appearing in Buu saga can be related to the prophecy.
