If he wanted to grow the rank of his dungeon, he needed a steady supply of DP and adventurers to enter his dungeon. Additionally, given that the enemy he would have to face in the future would only be stronger, he needed DP to add new floors, monsters and install various other traps and mechanisms to his dungeon.
Thus the idea that he spawns a monster that could potentially wipe out a low ranking party on the upper floors, didn't seem appropriate to him.
"I know what lord Simon is thinking, it is dangerous to spawn them on the upper floors but what about the middle floors? The adventurers there might be able to do something and come up with ways and strategies to defeat these monsters".
