The clan box comes every 2000 wins. It’s not enough incentive to keep the arena populated.
The point is to encourage players to return to arena as often as possible. By the way, this applies to arena wins.
Also, not everyone belongs to a clan, and some clans are lucky to get to 1000 wins for the week.
There needs to be some mechanism in place to boost the arena population to 1000 online players at any given stage.