When the Marvel Cinematic Universe introduced variants, it also showed the different lives that each one lives in another reality. For Kang the Conqueror, he is exiled in the Quantum Realm which is why he needs Scott Lang in Ant-Man & the Wasp: Quantumania but why did the Council of Kangs imprisoned him there?
Warning: This article contains spoilers for Ant-Man & the Wasp: Quantumania. Read at your own risk!
Why Did The Council of Kangs Exile Kang The Conqueror in Ant-Man & the Wasp: Quantumania?
It is one of the worst kinds of betrayal, the betrayal of one's self, but is that even applicable to Kang the Conqueror in Ant-Man & the Wasp: Quantumania considering that it is his own kind, his variants, that imprisoned him in the Quantum Realm?
See, each of the variants of a character has a different story, a different narrative and for Kang the Conqueror, he got the moniker of "the Conqueror," because he "conquered," or destroyed the realities of his other variants so that it could only be him in the end.
Janet Van Dyne saw this when she accidentally touched the power core of Kang's ship, one that is connected to his brain, and Janet knew then that she could not let Kang leave because he was imprisoned in the Quantum Realm for a reason -- he does not care about the lives he kill in his eradication of his variants.
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Why is the Quantum Realm Chosen as Kang The Conqueror's Prison in Ant-Man & the Wasp: Quantumania?
Simple, the Quantum Realm is a microuniverse of its own. It is a place beyond space and time, and without those two things, Kang the Conqueror would not be able to access other timelines or realities and destroy it just so he can become the supreme version of himself.
The Council of Kangs are bright enough to trap Kang in the Quantum Realm because since it exists beyond space and time, and with their plot of sabotaging his ship effective in Ant-Man & the Wasp: Quantumania, they need not worry about him getting out. Add to that, Ant-Man and the Wasp ended him.
The wonders of the Quantum Realm continue to leave the MCU fans in awe about it in Ant-Man & the Wasp: Quantumania, now in theaters worldwide.