Dark Night of the Imperium
The Arena is a favourite pastime shared by Middle hive workers and upper hive nobles alike. While the middle hivers come to cheer on their favourite fighters, the nobles play a game within a game. For the nobles, the outcome of the fights are less important than the outcome of a game played behind the scenes. For some nobles, the real thrill is in exercising their power to influence the outcome of various matches. Whether or not a particular fighter wins is entirely down to who can flex the most money, and wield enough power.
The fighters themselves are all condemned criminals. In theory, they are sentenced to fight in the Arena until they manage to win a certain number of fights at the top level. Most never make it that far, and for those that do, the meddling of nobles in the outcomes of the fights mean they will almost certainly never win the required number of fights.