Well, the actual reason is that Rich never intended the comic to be taken seriously at that point like it is now - it was just a joke-a-day about D&D for people browsing his gaming articles. It retroactively became a part of this huge plot about the gates.

But we can construe the in-universe explanation to be that Xykon/Redcloak cleared out most of the actual threats in the dungeon before the Order even got there.