The revenge arc is happening quickly. Maybe the author wants to get it out of the way to just focus the plot around..... whatever the plot is (like expanding the demon lords power or just slow life dungeon stuff; idk). Since they’ve already showed up, I would personally prefer a confrontation and them dying to a cheap escape and them spreading the information. I really don’t need that plot needlessly dragged out. If it was brought up early it needs to be dealt with early; otherwise the author should have kept focusing on the dungeon and not switched to traitor party until at least chapter 7 (I’d say 10 really; have some other adventures go first and die. I don’t even understand why they are paying such a high reward for a completely unknown dungeon).