The entire dungeon system was developed trying to balance reward and effort.
It was brought into the game to give players more ways of acquiring high Tiered items. However, the devs didn't want these items to flood the market. For more reasons than simple game balance.
As such, there were rewards for individuals and the group. However, that still left one problem. Increasing your party size would have made things too easy.
There was a limit of 20 players (or NPCs, indeed) for each party. However, dungeons allowed multiple parties to band together to be conquered. After all, some, like this dungeon, required at least a hundred players. Though a thousand was more realistic.
But, that meant there were more chest drops. More chances for the rare loot drops.
