Perhaps the dungeon is testing and developing something we just don't see immediately, but which might actually benefit from such weak monsters. Like logistics. If there are some rules to how the dungeon can create monsters (how powerful, how many, how fast, on which floors), the dungeon could be looking for ways to stretch some limitations. Or maybe the dungeon is looking for weird mutations, such as intelligence. It might need to go through a thousand specimens for something to appear. If the monsters were tough and few, it would take a long time to see result. If they are numerous and are replaced rapidly, more test subjects could be observed in much shorter time.

Was the xenos origins mentioned? If it wasn't, maybe they are such aberrations that had enough wits to leave the colosseum, instead of sticking around to fight to death.