Naturally, Damien wasn't going to give Matthias an entire cardinal region. It was stupid for him to even consider doing so.
Having sovereignty over a cardinal region came hand in hand with power. The reason the eight great clans were able to maintain their status without competitors for millions of years was precisely because of their status.
The resources they had access to, whether from the environment or the people, were monstrous. That went without even mentioning secret realms and the likes which would become the sovereign's property.
Those who served a great clan would gain benefits unlike any other, therefore, they'd always be the main choice for anyone who wanted to be someone, which meant geniuses would gravitate towards them without considering others first.
No matter what, Damien was keeping the regions he conquered.
The Kyushu Federation wasn't an influence that really craved land or power, but they did crave resources.