In a way, The Rule of Power was just a very important battle. Sure, it was a lot more complicated than that, but in the end, it was the battlefield that decided everything.
If Adam wins, Osana will be free.
If Adam loses, Osana will accept her fate.
Obviously, no one was going to fight in the banquet hall. It was an inappropriate place, deadly for the guests, and... there was no Adam's opponent here.
The Marzo Family needed to find a suitable opponent for Adam in the shortest possible terms, but they had some time to prepare.
It was up to the Marzo Family to find a second contestant since Adam took the liberty of using The Rule of Power. However, they also required an arena.
Arenas were not the only places where experienced fighters fought to satisfy their bloodlust or to have a casual fight with an old friend. They were places where significant matters settled with a primitive but revealing tool: power.