Simply because he currently CAN'T OP them. He's dragging time till when he finally could.
Also it has been mentioned that only killing the council members doesn't end it. In fact, it would cause people -especially the temple devotees - to riot against the king instead. So he needs to play his game right.
he doesn’t need to kill them to win. They have years of documented evidence of treason against the royal blood line, use that, quite literally you can do anything n more, plus it’s easy to convince the people because the ppl believe in the dragon and what is more evident than a king who wields the power of the dragon. Also he does OP them currently, maybe not then but rn he has the advantage.
1. The people fear the power of the dragon as they believe the king would go berserk without the council's "guidance".
2. The recordings about the council's doings are not official records. They're Abel's sect's recordings which aren't reliable from a jurisdical point of view regardless of their credibility.
3. The majority of the people support the council as they're the front image or the nation. Council members control the temples, which in their turn indoctrinate people to believe certain things, including that all kings would lose their sanity if it weren't for the council.
4. As a facade, the council supports the king. In reality, they control him and mold him to become useless so they can be the real rulers. If a king was to randomly claim that he's being poisoned = instant "mad" title.
5. Again, Karas still needs some time before he reaches his full power potential on the day of the dragon. He needs to stall for that day to arrive because he can't OP them right now.

I just finished this in one sitting and the limitations opposed on the King makes no sense to me. Why is he willingly playing a game with the council when he completely OPs them. Secondly, you’re the king who has the power of a DRAGON, why are you drinking the potion in the first place. Just be like no and gtfo.