It's also a question of dungeon design. The dungeons, as they are now, are designed to 4 persons and towers for 3 alliances, some more pronouncedly than others. (Halatali hm is a good example of exacty 4 persons in mind design) New content could of course be designed for different amount of players in a party but the old one is not necessarily as easy to change as "just increase the mob hp".