Bibidi sealed Buu away after Buu absorbed the Grand Supreme Kai. The Dragonball Wikia page for Majin Buu notes that he becomes easier to control after he absorbs the Grand Supreme Kai:
Of note, absorbing Grand Supreme Kai granted Majin Buu the ability to speak fluently, although he usually refers to himself in the third person. Buu became calmer and easier to control after absorbing the Grand Supreme Kai, and Bibidi then decided to seal him in a Sealed Ball.
Why would Buu need to be sealed up if he had become easier to control?