Why isn't Supreme Kai stronger?

[Hellspawn28]

He is a god. He is suppose to natural and not to interfere with mortals unless they are a huge threat like Majin Buu. He also probably didn't think anyone above Buu would exist.

[dragon boss z]

Also I know there is no official canon, but using a video game story is going too far. Even filler holds more weight.

Well... no. Xenoverse is about as logical a DB game as you can get, aside from Online (as I'd presume). You can pretty easily use Demigra as an example in Kaiōshin's situation.

Demigra isn't even a kaioshin though he is a makaioshin from what I have gathered. And Xenoverse isn't written by Toriyama so I wouldn't cross the two over. Also Zamasu seemed to get strong quick. It just comes down to the Supreme Kai not really being much of a fighter (compared to other gods since he was still above Frieza).

[Nejishiki]

Boo's release was hardly Kaioshin's "fault". Goku and Vegeta, the main culprits, intentionally donated energy for him. They believed Kaioshin exaggerated his strength based on the quality of Bobbodi's warriors. Kaioshin's fear was related to shocking Boo awake. That's why he was cautious. He wanted them defeated at once instead of potentially adding energy. At this point, Gohan already contributed a large sum. It's why he left the ball alone in the first place. He was too petrified of releasing him if destroying or relocating the ball went wrong. His fighting ability was never put into question by the narrative. It was his justified fear of Boo's monstrous strength that was tested.

Demigra is never referred to as a Makaioshin in any reputable source. As it currently stands, there are no Makaioshin characters in Dragon Ball. As for Zamasu, he grew in power through his Goku Black body while Future Zamasu relied on his immortality, usually with no regard for his defense. Aside from him being an expert martial artist, there's not enough supporting details to to gauge how quickly he grows on his own. I think feeling that he needed Goku's strength to progress says enough.

[buutenks]

The Supreme Kai was written to be a weak god and incompetent.

The more competent Kai's got killed off by Buu. One of the Kai's was so strong kid buu had to resort to absorbing him, since Buu couldn't defeat the Kai with raw power.