Demons Are Selfish.

The demon dukes can't go all out because of their suspicion and it is going to make them lose. The demon lord needs to be overwhelmed early on before she gets strong enough that she can eat their best effort. At this rate, the dukes will be the ones to succumb just like the demon nobles succumbed 10,000 years ago.

It will be a repeat of history. Hundreds of demon nobles had surrounded demon lord Beelta with more than 30 of them being prime nobles who could singlehandedly hold their own against her. Only 8 prime demons survived that war when their prey became strong enough to turn into their predator. Of those 8, none of them wanted to ever be around her with their bodies. They will only attend meetings with her with their divine sense instead of attending physically.

Baal shook his head and said, "Demons are selfish."