I think they like to stick to thier strengths when telling a believable story in the Yakuza universe. The Omi alliance has the most to gain from a Kamurocho take over not to mention it’s the only organisation besides the police that would make the story of Yakuza 5 plausible.
With Daigo trying to unite the clans of the other big cities to help take the Omi alliance head on it should be great trying to see these old rivals trying to make peace!